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Commits on Jul 22, 2021
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KVM: Consider SMT idle status when halt polling
SMT siblings share caches and other hardware, halt polling will degrade its sibling performance if its sibling is busy Signed-off-by: Li RongQing <lirongqing@baidu.com>
Commits on Jun 28, 2021
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wait: use LIST_HEAD_INIT() to initialize wait_queue_head
Replace the open-coded initialization with the right macro. Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20210601151120.329223-1-jwi@linux.ibm.com
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sched/debug: Don't update sched_domain debug directories before sched…
…_debug_init() Since CPU capacity asymmetry can stem purely from maximum frequency differences (e.g. Pixel 1), a rebuild of the scheduler topology can be issued upon loading cpufreq, see: arch_topology.c::init_cpu_capacity_callback() Turns out that if this rebuild happens *before* sched_debug_init() is run (which is a late initcall), we end up messing up the sched_domain debug directory: passing a NULL parent to debugfs_create_dir() ends up creating the directory at the debugfs root, which in this case creates /sys/kernel/debug/domains (instead of /sys/kernel/debug/sched/domains). This currently doesn't happen on asymmetric systems which use cpufreq-scpi or cpufreq-dt drivers, as those are loaded via deferred_probe_initcall() (it is also a late initcall, but appears to be ordered *after* sched_debug_init()). Ionela has been working on detecting maximum frequency asymmetry via ACPI, and that actually happens via a *device* initcall, thus before sched_debug_init(), and causes the aforementionned debugfs mayhem. One option would be to punt sched_debug_init() down to fs_initcall_sync(). Preventing update_sched_domain_debugfs() from running before sched_debug_init() appears to be the safer option. Fixes: 3b87f13 ("sched,debug: Convert sysctl sched_domains to debugfs") Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: http://lore.kernel.org/r/20210514095339.12979-1-ionela.voinescu@arm.com
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sched/fair: Ensure _sum and _avg values stay consistent
The _sum and _avg values are in general sync together with the PELT divider. They are however not always completely in perfect sync, resulting in situations where _sum gets to zero while _avg stays positive. Such situations are undesirable. This comes from the fact that PELT will increase period_contrib, also increasing the PELT divider, without updating _sum and _avg values to stay in perfect sync where (_sum == _avg * divider). However, such PELT change will never lower _sum, making it impossible to end up in a situation where _sum is zero and _avg is not. Therefore, we need to ensure that when subtracting load outside PELT, that when _sum is zero, _avg is also set to zero. This occurs when (_sum < _avg * divider), and the subtracted (_avg * divider) is bigger or equal to the current _sum, while the subtracted _avg is smaller than the current _avg. Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org> Signed-off-by: Odin Ugedal <odin@uged.al> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com> Link: https://lore.kernel.org/r/20210624111815.57937-1-odin@uged.al
Commits on Jun 24, 2021
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sched/doc: Update the CPU capacity asymmetry bits
Update the documentation bits referring to capacity aware scheduling with regards to newly introduced SD_ASYM_CPUCAPACITY_FULL sched_domain flag. Signed-off-by: Beata Michalska <beata.michalska@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Link: https://lore.kernel.org/r/20210603140627.8409-4-beata.michalska@arm.com
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sched/topology: Rework CPU capacity asymmetry detection
Currently the CPU capacity asymmetry detection, performed through asym_cpu_capacity_level, tries to identify the lowest topology level at which the highest CPU capacity is being observed, not necessarily finding the level at which all possible capacity values are visible to all CPUs, which might be bit problematic for some possible/valid asymmetric topologies i.e.: DIE [ ] MC [ ][ ] CPU [0] [1] [2] [3] [4] [5] [6] [7] Capacity |.....| |.....| |.....| |.....| L M B B Where: arch_scale_cpu_capacity(L) = 512 arch_scale_cpu_capacity(M) = 871 arch_scale_cpu_capacity(B) = 1024 In this particular case, the asymmetric topology level will point at MC, as all possible CPU masks for that level do cover the CPU with the highest capacity. It will work just fine for the first cluster, not so much for the second one though (consider the find_energy_efficient_cpu which might end up attempting the energy aware wake-up for a domain that does not see any asymmetry at all) Rework the way the capacity asymmetry levels are being detected, allowing to point to the lowest topology level (for a given CPU), where full set of available CPU capacities is visible to all CPUs within given domain. As a result, the per-cpu sd_asym_cpucapacity might differ across the domains. This will have an impact on EAS wake-up placement in a way that it might see different range of CPUs to be considered, depending on the given current and target CPUs. Additionally, those levels, where any range of asymmetry (not necessarily full) is being detected will get identified as well. The selected asymmetric topology level will be denoted by SD_ASYM_CPUCAPACITY_FULL sched domain flag whereas the 'sub-levels' would receive the already used SD_ASYM_CPUCAPACITY flag. This allows maintaining the current behaviour for asymmetric topologies, with misfit migration operating correctly on lower levels, if applicable, as any asymmetry is enough to trigger the misfit migration. The logic there relies on the SD_ASYM_CPUCAPACITY flag and does not relate to the full asymmetry level denoted by the sd_asym_cpucapacity pointer. Detecting the CPU capacity asymmetry is being based on a set of available CPU capacities for all possible CPUs. This data is being generated upon init and updated once CPU topology changes are being detected (through arch_update_cpu_topology). As such, any changes to identified CPU capacities (like initializing cpufreq) need to be explicitly advertised by corresponding archs to trigger rebuilding the data. Additional -dflags- parameter, used when building sched domains, has been removed as well, as the asymmetry flags are now being set directly in sd_init. Suggested-by: Peter Zijlstra <peterz@infradead.org> Suggested-by: Valentin Schneider <valentin.schneider@arm.com> Signed-off-by: Beata Michalska <beata.michalska@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Tested-by: Valentin Schneider <valentin.schneider@arm.com> Link: https://lore.kernel.org/r/20210603140627.8409-3-beata.michalska@arm.com
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sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched_domain flag
Introducing new, complementary to SD_ASYM_CPUCAPACITY, sched_domain topology flag, to distinguish between shed_domains where any CPU capacity asymmetry is detected (SD_ASYM_CPUCAPACITY) and ones where a full set of CPU capacities is visible to all domain members (SD_ASYM_CPUCAPACITY_FULL). With the distinction between full and partial CPU capacity asymmetry, brought in by the newly introduced flag, the scope of the original SD_ASYM_CPUCAPACITY flag gets shifted, still maintaining the existing behaviour when one is detected on a given sched domain, allowing misfit migrations within sched domains that do not observe full range of CPU capacities but still do have members with different capacity values. It loses though it's meaning when it comes to the lowest CPU asymmetry sched_domain level per-cpu pointer, which is to be now denoted by SD_ASYM_CPUCAPACITY_FULL flag. Signed-off-by: Beata Michalska <beata.michalska@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Link: https://lore.kernel.org/r/20210603140627.8409-2-beata.michalska@arm.com
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sched/fair: Introduce the burstable CFS controller
The CFS bandwidth controller limits CPU requests of a task group to quota during each period. However, parallel workloads might be bursty so that they get throttled even when their average utilization is under quota. And they are latency sensitive at the same time so that throttling them is undesired. We borrow time now against our future underrun, at the cost of increased interference against the other system users. All nicely bounded. Traditional (UP-EDF) bandwidth control is something like: (U = \Sum u_i) <= 1 This guaranteeds both that every deadline is met and that the system is stable. After all, if U were > 1, then for every second of walltime, we'd have to run more than a second of program time, and obviously miss our deadline, but the next deadline will be further out still, there is never time to catch up, unbounded fail. This work observes that a workload doesn't always executes the full quota; this enables one to describe u_i as a statistical distribution. For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100) (the traditional WCET). This effectively allows u to be smaller, increasing the efficiency (we can pack more tasks in the system), but at the cost of missing deadlines when all the odds line up. However, it does maintain stability, since every overrun must be paired with an underrun as long as our x is above the average. That is, suppose we have 2 tasks, both specify a p(95) value, then we have a p(95)*p(95) = 90.25% chance both tasks are within their quota and everything is good. At the same time we have a p(5)p(5) = 0.25% chance both tasks will exceed their quota at the same time (guaranteed deadline fail). Somewhere in between there's a threshold where one exceeds and the other doesn't underrun enough to compensate; this depends on the specific CDFs. At the same time, we can say that the worst case deadline miss, will be \Sum e_i; that is, there is a bounded tardiness (under the assumption that x+e is indeed WCET). The benefit of burst is seen when testing with schbench. Default value of kernel.sched_cfs_bandwidth_slice_us(5ms) and CONFIG_HZ(1000) is used. mkdir /sys/fs/cgroup/cpu/test echo $$ > /sys/fs/cgroup/cpu/test/cgroup.procs echo 100000 > /sys/fs/cgroup/cpu/test/cpu.cfs_quota_us echo 100000 > /sys/fs/cgroup/cpu/test/cpu.cfs_burst_us ./schbench -m 1 -t 3 -r 20 -c 80000 -R 10 The average CPU usage is at 80%. I run this for 10 times, and got long tail latency for 6 times and got throttled for 8 times. Tail latencies are shown below, and it wasn't the worst case. Latency percentiles (usec) 50.0000th: 19872 75.0000th: 21344 90.0000th: 22176 95.0000th: 22496 *99.0000th: 22752 99.5000th: 22752 99.9000th: 22752 min=0, max=22727 rps: 9.90 p95 (usec) 22496 p99 (usec) 22752 p95/cputime 28.12% p99/cputime 28.44% The interferenece when using burst is valued by the possibilities for missing the deadline and the average WCET. Test results showed that when there many cgroups or CPU is under utilized, the interference is limited. More details are shown in: https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/ Co-developed-by: Shanpei Chen <shanpeic@linux.alibaba.com> Signed-off-by: Shanpei Chen <shanpeic@linux.alibaba.com> Co-developed-by: Tianchen Ding <dtcccc@linux.alibaba.com> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com> Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Ben Segall <bsegall@google.com> Acked-by: Tejun Heo <tj@kernel.org> Link: https://lore.kernel.org/r/20210621092800.23714-2-changhuaixin@linux.alibaba.com
Commits on Jun 22, 2021
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sched/uclamp: Fix uclamp_tg_restrict()
Now cpu.uclamp.min acts as a protection, we need to make sure that the uclamp request of the task is within the allowed range of the cgroup, that is it is clamp()'ed correctly by tg->uclamp[UCLAMP_MIN] and tg->uclamp[UCLAMP_MAX]. As reported by Xuewen [1] we can have some corner cases where there's inversion between uclamp requested by task (p) and the uclamp values of the taskgroup it's attached to (tg). Following table demonstrates 2 corner cases: | p | tg | effective -----------+-----+------+----------- CASE 1 -----------+-----+------+----------- uclamp_min | 60% | 0% | 60% -----------+-----+------+----------- uclamp_max | 80% | 50% | 50% -----------+-----+------+----------- CASE 2 -----------+-----+------+----------- uclamp_min | 0% | 30% | 30% -----------+-----+------+----------- uclamp_max | 20% | 50% | 20% -----------+-----+------+----------- With this fix we get: | p | tg | effective -----------+-----+------+----------- CASE 1 -----------+-----+------+----------- uclamp_min | 60% | 0% | 50% -----------+-----+------+----------- uclamp_max | 80% | 50% | 50% -----------+-----+------+----------- CASE 2 -----------+-----+------+----------- uclamp_min | 0% | 30% | 30% -----------+-----+------+----------- uclamp_max | 20% | 50% | 30% -----------+-----+------+----------- Additionally uclamp_update_active_tasks() must now unconditionally update both UCLAMP_MIN/MAX because changing the tg's UCLAMP_MAX for instance could have an impact on the effective UCLAMP_MIN of the tasks. | p | tg | effective -----------+-----+------+----------- old -----------+-----+------+----------- uclamp_min | 60% | 0% | 50% -----------+-----+------+----------- uclamp_max | 80% | 50% | 50% -----------+-----+------+----------- *new* -----------+-----+------+----------- uclamp_min | 60% | 0% | *60%* -----------+-----+------+----------- uclamp_max | 80% |*70%* | *70%* -----------+-----+------+----------- [1] https://lore.kernel.org/lkml/CAB8ipk_a6VFNjiEnHRHkUMBKbA+qzPQvhtNjJ_YNzQhqV_o8Zw@mail.gmail.com/ Fixes: 0c18f2e ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min") Reported-by: Xuewen Yan <xuewen.yan94@gmail.com> Signed-off-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20210617165155.3774110-1-qais.yousef@arm.com
Commits on Jun 18, 2021
Commits on Jun 17, 2021
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sched/cpufreq: Consider reduced CPU capacity in energy calculation
Energy Aware Scheduling (EAS) needs to predict the decisions made by SchedUtil. The map_util_freq() exists to do that. There are corner cases where the max allowed frequency might be reduced (due to thermal). SchedUtil as a CPUFreq governor, is aware of that but EAS is not. This patch aims to address it. SchedUtil stores the maximum allowed frequency in 'sugov_policy::next_freq' field. EAS has to predict that value, which is the real used frequency. That value is made after a call to cpufreq_driver_resolve_freq() which clamps to the CPUFreq policy limits. In the existing code EAS is not able to predict that real frequency. This leads to energy estimation errors. To avoid wrong energy estimation in EAS (due to frequency miss prediction) make sure that the step which calculates Performance Domain frequency, is also aware of the allowed CPU capacity. Furthermore, modify map_util_freq() to not extend the frequency value. Instead, use map_util_perf() to extend the util value in both places: SchedUtil and EAS, but for EAS clamp it to max allowed CPU capacity. In the end, we achieve the same desirable behavior for both subsystems and alignment in regards to the real CPU frequency. Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> (For the schedutil part) Link: https://lore.kernel.org/r/20210614191238.23224-1-lukasz.luba@arm.com
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sched/fair: Take thermal pressure into account while estimating energy
Energy Aware Scheduling (EAS) needs to be able to predict the frequency requests made by the SchedUtil governor to properly estimate energy used in the future. It has to take into account CPUs utilization and forecast Performance Domain (PD) frequency. There is a corner case when the max allowed frequency might be reduced due to thermal. SchedUtil is aware of that reduced frequency, so it should be taken into account also in EAS estimations. SchedUtil, as a CPUFreq governor, knows the maximum allowed frequency of a CPU, thanks to cpufreq_driver_resolve_freq() and internal clamping to 'policy::max'. SchedUtil is responsible to respect that upper limit while setting the frequency through CPUFreq drivers. This effective frequency is stored internally in 'sugov_policy::next_freq' and EAS has to predict that value. In the existing code the raw value of arch_scale_cpu_capacity() is used for clamping the returned CPU utilization from effective_cpu_util(). This patch fixes issue with too big single CPU utilization, by introducing clamping to the allowed CPU capacity. The allowed CPU capacity is a CPU capacity reduced by thermal pressure raw value. Thanks to knowledge about allowed CPU capacity, we don't get too big value for a single CPU utilization, which is then added to the util sum. The util sum is used as a source of information for estimating whole PD energy. To avoid wrong energy estimation in EAS (due to capped frequency), make sure that the calculation of util sum is aware of allowed CPU capacity. This thermal pressure might be visible in scenarios where the CPUs are not heavily loaded, but some other component (like GPU) drastically reduced available power budget and increased the SoC temperature. Thus, we still use EAS for task placement and CPUs are not over-utilized. Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Link: https://lore.kernel.org/r/20210614191128.22735-1-lukasz.luba@arm.com
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thermal/cpufreq_cooling: Update offline CPUs per-cpu thermal_pressure
The thermal pressure signal gives information to the scheduler about reduced CPU capacity due to thermal. It is based on a value stored in a per-cpu 'thermal_pressure' variable. The online CPUs will get the new value there, while the offline won't. Unfortunately, when the CPU is back online, the value read from per-cpu variable might be wrong (stale data). This might affect the scheduler decisions, since it sees the CPU capacity differently than what is actually available. Fix it by making sure that all online+offline CPUs would get the proper value in their per-cpu variable when thermal framework sets capping. Fixes: f12e4f6 ("thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping") Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Viresh Kumar <viresh.kumar@linaro.org> Link: https://lore.kernel.org/r/20210614191030.22241-1-lukasz.luba@arm.com
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sched/fair: Return early from update_tg_cfs_load() if delta == 0
In case the _avg delta is 0 there is no need to update se's _avg (level n) nor cfs_rq's _avg (level n-1). These values stay the same. Since cfs_rq's _avg isn't changed, i.e. no load is propagated down, cfs_rq's _sum should stay the same as well. So bail out after se's _sum has been updated. Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Link: https://lore.kernel.org/r/20210601083616.804229-1-dietmar.eggemann@arm.com
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sched/pelt: Check that *_avg are null when *_sum are
Check that we never break the rule that pelt's avg values are null if pelt's sum are. Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Acked-by: Odin Ugedal <odin@uged.al> Link: https://lore.kernel.org/r/20210601155328.19487-1-vincent.guittot@linaro.org
Commits on Jun 14, 2021
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sched/fair: Correctly insert cfs_rq's to list on unthrottle
Fix an issue where fairness is decreased since cfs_rq's can end up not being decayed properly. For two sibling control groups with the same priority, this can often lead to a load ratio of 99/1 (!!). This happens because when a cfs_rq is throttled, all the descendant cfs_rq's will be removed from the leaf list. When they initial cfs_rq is unthrottled, it will currently only re add descendant cfs_rq's if they have one or more entities enqueued. This is not a perfect heuristic. Instead, we insert all cfs_rq's that contain one or more enqueued entities, or it its load is not completely decayed. Can often lead to situations like this for equally weighted control groups: $ ps u -C stress USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 10009 88.8 0.0 3676 100 pts/1 R+ 11:04 0:13 stress --cpu 1 root 10023 3.0 0.0 3676 104 pts/1 R+ 11:04 0:00 stress --cpu 1 Fixes: 31bc6ae ("sched/fair: Optimize update_blocked_averages()") [vingo: !SMP build fix] Signed-off-by: Odin Ugedal <odin@uged.al> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Link: https://lore.kernel.org/r/20210612112815.61678-1-odin@uged.al
Commits on Jun 13, 2021
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Merge tag 'perf-tools-fixes-for-v5.13-2021-06-13' of git://git.kernel…
….org/pub/scm/linux/kernel/git/acme/linux Pull perf tools fixes from Arnaldo Carvalho de Melo: - Correct buffer copying when peeking events - Sync cpufeatures/disabled-features.h header with the kernel sources * tag 'perf-tools-fixes-for-v5.13-2021-06-13' of git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux: tools headers cpufeatures: Sync with the kernel sources perf session: Correct buffer copying when peeking events
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Merge tag 'nfs-for-5.13-3' of git://git.linux-nfs.org/projects/trondm…
…y/linux-nfs Pull NFS client bugfixes from Trond Myklebust: "Highlights include: Stable fixes: - Fix use-after-free in nfs4_init_client() Bugfixes: - Fix deadlock between nfs4_evict_inode() and nfs4_opendata_get_inode() - Fix second deadlock in nfs4_evict_inode() - nfs4_proc_set_acl should not change the value of NFS_CAP_UIDGID_NOMAP - Fix setting of the NFS_CAP_SECURITY_LABEL capability" * tag 'nfs-for-5.13-3' of git://git.linux-nfs.org/projects/trondmy/linux-nfs: NFSv4: Fix second deadlock in nfs4_evict_inode() NFSv4: Fix deadlock between nfs4_evict_inode() and nfs4_opendata_get_inode() NFS: FMODE_READ and friends are C macros, not enum types NFS: Fix a potential NULL dereference in nfs_get_client() NFS: Fix use-after-free in nfs4_init_client() NFS: Ensure the NFS_CAP_SECURITY_LABEL capability is set when appropriate NFSv4: nfs4_proc_set_acl needs to restore NFS_CAP_UIDGID_NOMAP on error.
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Merge tag 'scsi-fixes' of git://git.kernel.org/pub/scm/linux/kernel/g…
…it/jejb/scsi Pull SCSI fixes from James Bottomley: "Four reasonably small fixes to the core for scsi host allocation failure paths. The root problem is that we're not freeing the memory allocated by dev_set_name(), which involves a rejig of may of the free on error paths to do put_device() instead of kfree which, in turn, has several other knock on ramifications and inspection turned up a few other lurking bugs" * tag 'scsi-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi: scsi: core: Only put parent device if host state differs from SHOST_CREATED scsi: core: Put .shost_dev in failure path if host state changes to RUNNING scsi: core: Fix failure handling of scsi_add_host_with_dma() scsi: core: Fix error handling of scsi_host_alloc()
Commits on Jun 12, 2021
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Merge tag 'riscv-for-linus-5.13-rc6' of git://git.kernel.org/pub/scm/…
…linux/kernel/git/riscv/linux Pull RISC-V fixes from Palmer Dabbelt: - A pair of XIP fixes: one to fix alternatives, and one to turn off the rest of the features that require code modification - A fix to a type that was causing some alternatives to break - A build fix for BUILTIN_DTB * tag 'riscv-for-linus-5.13-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux: riscv: Fix BUILTIN_DTB for sifive and microchip soc riscv: alternative: fix typo in macro name riscv: code patching only works on !XIP_KERNEL riscv: xip: support runtime trap patching
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mm: relocate 'write_protect_seq' in struct mm_struct
0day robot reported a 9.2% regression for will-it-scale mmap1 test case[1], caused by commit 57efa1f ("mm/gup: prevent gup_fast from racing with COW during fork"). Further debug shows the regression is due to that commit changes the offset of hot fields 'mmap_lock' inside structure 'mm_struct', thus some cache alignment changes. From the perf data, the contention for 'mmap_lock' is very severe and takes around 95% cpu cycles, and it is a rw_semaphore struct rw_semaphore { atomic_long_t count; /* 8 bytes */ atomic_long_t owner; /* 8 bytes */ struct optimistic_spin_queue osq; /* spinner MCS lock */ ... Before commit 57efa1f adds the 'write_protect_seq', it happens to have a very optimal cache alignment layout, as Linus explained: "and before the addition of the 'write_protect_seq' field, the mmap_sem was at offset 120 in 'struct mm_struct'. Which meant that count and owner were in two different cachelines, and then when you have contention and spend time in rwsem_down_write_slowpath(), this is probably *exactly* the kind of layout you want. Because first the rwsem_write_trylock() will do a cmpxchg on the first cacheline (for the optimistic fast-path), and then in the case of contention, rwsem_down_write_slowpath() will just access the second cacheline. Which is probably just optimal for a load that spends a lot of time contended - new waiters touch that first cacheline, and then they queue themselves up on the second cacheline." After the commit, the rw_semaphore is at offset 128, which means the 'count' and 'owner' fields are now in the same cacheline, and causes more cache bouncing. Currently there are 3 "#ifdef CONFIG_XXX" before 'mmap_lock' which will affect its offset: CONFIG_MMU CONFIG_MEMBARRIER CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES The layout above is on 64 bits system with 0day's default kernel config (similar to RHEL-8.3's config), in which all these 3 options are 'y'. And the layout can vary with different kernel configs. Relayouting a structure is usually a double-edged sword, as sometimes it can helps one case, but hurt other cases. For this case, one solution is, as the newly added 'write_protect_seq' is a 4 bytes long seqcount_t (when CONFIG_DEBUG_LOCK_ALLOC=n), placing it into an existing 4 bytes hole in 'mm_struct' will not change other fields' alignment, while restoring the regression. Link: https://lore.kernel.org/lkml/20210525031636.GB7744@xsang-OptiPlex-9020/ [1] Reported-by: kernel test robot <oliver.sang@intel.com> Signed-off-by: Feng Tang <feng.tang@intel.com> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Cc: Peter Xu <peterx@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
