From 0d61a651e4dd3c61d1658cc92e0b0450c8374738 Mon Sep 17 00:00:00 2001 From: Charan Teja Reddy Date: Fri, 5 Feb 2021 17:47:57 +0530 Subject: [PATCH] ANDROID: vmscan: Support multiple kswapd threads per node Page replacement is handled in the Linux Kernel in one of two ways: 1) Asynchronously via kswapd 2) Synchronously, via direct reclaim At page allocation time the allocating task is immediately given a page from the zone free list allowing it to go right back to work doing whatever it was doing; Probably directly or indirectly executing business logic. Just prior to satisfying the allocation, free pages is checked to see if it has reached the zone low watermark and if so, kswapd is awakened. Kswapd will start scanning pages looking for inactive pages to evict to make room for new page allocations. The work of kswapd allows tasks to continue allocating memory from their respective zone free list without incurring any delay. When the demand for free pages exceeds the rate that kswapd tasks can supply them, page allocation works differently. Once the allocating task finds that the number of free pages is at or below the zone min watermark, the task will no longer pull pages from the free list. Instead, the task will run the same CPU-bound routines as kswapd to satisfy its own allocation by scanning and evicting pages. This is called a direct reclaim. The time spent performing a direct reclaim can be substantial, often taking tens to hundreds of milliseconds for small order0 allocations to half a second or more for order9 huge-page allocations. In fact, kswapd is not actually required on a linux system. It exists for the sole purpose of optimizing performance by preventing direct reclaims. When memory shortfall is sufficient to trigger direct reclaims, they can occur in any task that is running on the system. A single aggressive memory allocating task can set the stage for collateral damage to occur in small tasks that rarely allocate additional memory. Consider the impact of injecting an additional 100ms of latency when nscd allocates memory to facilitate caching of a DNS query. The presence of direct reclaims 10 years ago was a fairly reliable indicator that too much was being asked of a Linux system. Kswapd was likely wasting time scanning pages that were ineligible for eviction. Adding RAM or reducing the working set size would usually make the problem go away. Since then hardware has evolved to bring a new struggle for kswapd. Storage speeds have increased by orders of magnitude while CPU clock speeds stayed the same or even slowed down in exchange for more cores per package. This presents a throughput problem for a single threaded kswapd that will get worse with each generation of new hardware. Test Details NOTE: The tests below were run with shadow entries disabled. See the associated patch and cover letter for details The tests below were designed with the assumption that a kswapd bottleneck is best demonstrated using filesystem reads. This way, the inactive list will be full of clean pages, simplifying the analysis and allowing kswapd to achieve the highest possible steal rate. Maximum steal rates for kswapd are likely to be the same or lower for any other mix of page types on the system. Tests were run on a 2U Oracle X7-2L with 52 Intel Xeon Skylake 2GHz cores, 756GB of RAM and 8 x 3.6 TB NVMe Solid State Disk drives. Each drive has an XFS file system mounted separately as /d0 through /d7. SSD drives require multiple concurrent streams to show their potential, so I created eleven 250GB zero-filled files on each drive so that I could test with parallel reads. The test script runs in multiple stages. At each stage, the number of dd tasks run concurrently is increased by 2. I did not include all of the test output for brevity. During each stage dd tasks are launched to read from each drive in a round robin fashion until the specified number of tasks for the stage has been reached. Then iostat, vmstat and top are started in the background with 10 second intervals. After five minutes, all of the dd tasks are killed and the iostat, vmstat and top output is parsed in order to report the following: CPU consumption - sy - aggregate kernel mode CPU consumption from vmstat output. The value doesn't tend to fluctuate much so I just grab the highest value. Each sample is averaged over 10 seconds - dd_cpu - for all of the dd tasks averaged across the top samples since there is a lot of variation. Throughput - in Kbytes - Command is iostat -x -d 10 -g total This first test performs reads using O_DIRECT in order to show the maximum throughput that can be obtained using these drives. It also demonstrates how rapidly throughput scales as the number of dd tasks are increased. The dd command for this test looks like this: Command Used: dd iflag=direct if=/d${i}/$n of=/dev/null bs=4M Test #1: Direct IO dd sy dd_cpu throughput 6 0 2.33 14726026.40 10 1 2.95 19954974.80 16 1 2.63 24419689.30 22 1 2.63 25430303.20 28 1 2.91 26026513.20 34 1 2.53 26178618.00 40 1 2.18 26239229.20 46 1 1.91 26250550.40 52 1 1.69 26251845.60 58 1 1.54 26253205.60 64 1 1.43 26253780.80 70 1 1.31 26254154.80 76 1 1.21 26253660.80 82 1 1.12 26254214.80 88 1 1.07 26253770.00 90 1 1.04 26252406.40 Throughput was close to peak with only 22 dd tasks. Very little system CPU was consumed as expected as the drives DMA directly into the user address space when using direct IO. In this next test, the iflag=direct option is removed and we only run the test until the pgscan_kswapd from /proc/vmstat starts to increment. At that point metrics are parsed and reported and the pagecache contents are dropped prior to the next test. Lather, rinse, repeat. Test #2: standard file system IO, no page replacement dd sy dd_cpu throughput 6 2 28.78 5134316.40 10 3 31.40 8051218.40 16 5 34.73 11438106.80 22 7 33.65 14140596.40 28 8 31.24 16393455.20 34 10 29.88 18219463.60 40 11 28.33 19644159.60 46 11 25.05 20802497.60 52 13 26.92 22092370.00 58 13 23.29 22884881.20 64 14 23.12 23452248.80 70 15 22.40 23916468.00 76 16 22.06 24328737.20 82 17 20.97 24718693.20 88 16 18.57 25149404.40 90 16 18.31 25245565.60 Each read has to pause after the buffer in kernel space is populated while those pages are added to the pagecache and copied into the user address space. For this reason, more parallel streams are required to achieve peak throughput. The copy operation consumes substantially more CPU than direct IO as expected. The next test measures throughput after kswapd starts running. This is the same test only we wait for kswapd to wake up before we start collecting metrics. The script actually keeps track of a few things that were not mentioned earlier. It tracks direct reclaims and page scans by watching the metrics in /proc/vmstat. CPU consumption for kswapd is tracked the same way it is tracked for dd. Since the test is 100% reads, you can assume that the page steal rate for kswapd and direct reclaims is almost identical to the scan rate. Test #3: 1 kswapd thread per node dd sy dd_cpu kswapd0 kswapd1 throughput dr pgscan_kswapd pgscan_direct 10 4 26.07 28.56 27.03 7355924.40 0 459316976 0 16 7 34.94 69.33 69.66 10867895.20 0 872661643 0 22 10 36.03 93.99 99.33 13130613.60 489 1037654473 11268334 28 10 30.34 95.90 98.60 14601509.60 671 1182591373 15429142 34 14 34.77 97.50 99.23 16468012.00 10850 1069005644 249839515 40 17 36.32 91.49 97.11 17335987.60 18903 975417728 434467710 46 19 38.40 90.54 91.61 17705394.40 25369 855737040 582427973 52 22 40.88 83.97 83.70 17607680.40 31250 709532935 724282458 58 25 40.89 82.19 80.14 17976905.60 35060 657796473 804117540 64 28 41.77 73.49 75.20 18001910.00 39073 561813658 895289337 70 33 45.51 63.78 64.39 17061897.20 44523 379465571 1020726436 76 36 46.95 57.96 60.32 16964459.60 47717 291299464 1093172384 82 39 47.16 55.43 56.16 16949956.00 49479 247071062 1134163008 88 42 47.41 53.75 47.62 16930911.20 51521 195449924 1180442208 90 43 47.18 51.40 50.59 16864428.00 51618 190758156 1183203901 In the previous test where kswapd was not involved, the system-wide kernel mode CPU consumption with 90 dd tasks was 16%. In this test CPU consumption with 90 tasks is at 43%. With 52 cores, and two kswapd tasks (one per NUMA node), kswapd can only be responsible for a little over 4% of the increase. The rest is likely caused by 51,618 direct reclaims that scanned 1.2 billion pages over the five minute time period of the test. Same test, more kswapd tasks: Test #4: 4 kswapd threads per node dd sy dd_cpu kswapd0 kswapd1 throughput dr pgscan_kswapd pgscan_direct 10 5 27.09 16.65 14.17 7842605.60 0 459105291 0 16 10 37.12 26.02 24.85 11352920.40 15 920527796 358515 22 11 36.94 37.13 35.82 13771869.60 0 1132169011 0 28 13 35.23 48.43 46.86 16089746.00 0 1312902070 0 34 15 33.37 53.02 55.69 18314856.40 0 1476169080 0 40 19 35.90 69.60 64.41 19836126.80 0 1629999149 0 46 22 36.82 88.55 57.20 20740216.40 0 1708478106 0 52 24 34.38 93.76 68.34 21758352.00 0 1794055559 0 58 24 30.51 79.20 82.33 22735594.00 0 1872794397 0 64 26 30.21 97.12 76.73 23302203.60 176 1916593721 4206821 70 33 32.92 92.91 92.87 23776588.00 3575 1817685086 85574159 76 37 31.62 91.20 89.83 24308196.80 4752 1812262569 113981763 82 29 25.53 93.23 92.33 24802791.20 306 2032093122 7350704 88 43 37.12 76.18 77.01 25145694.40 20310 1253204719 487048202 90 42 38.56 73.90 74.57 22516787.60 22774 1193637495 545463615 By increasing the number of kswapd threads, throughput increased by ~50% while kernel mode CPU utilization decreased or stayed the same, likely due to a decrease in the number of parallel tasks at any given time doing page replacement. Signed-off-by: Buddy Lumpkin Bug: 171351667 Link: https://lore.kernel.org/lkml/1522661062-39745-1-git-send-email-buddy.lumpkin@oracle.com [charante@codeaurora.org]: Changes made to select number of kswapds through uapi Change-Id: I8425cab7f40cbeaf65af0ea118c1a9ac7da0930e Signed-off-by: Charan Teja Reddy --- .../admin-guide/kernel-parameters.txt | 20 ++++++ include/linux/mmzone.h | 3 + mm/vmscan.c | 65 +++++++++++++++++++ 3 files changed, 88 insertions(+) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 75a717c038008..bd964e5b395f1 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2971,6 +2971,26 @@ firmware feature for updating multiple TCE entries at a time. + kswapd_per_node= + kswapd_per_node allows you to control the number of kswapd threads + running on the system. This provides the ability to devote additional + CPU resources toward proactive page replacement with the goal of + reducing direct reclaims. When direct reclaims are prevented, the CPU + consumed by them is prevented as well. Depending on the workload, the + result can cause aggregate CPU usage on the system to go up, down or + stay the same. + + More aggressive page replacement can reduce direct reclaims which + cause latency for tasks and decrease throughput when doing filesystem + IO through the pagecache. Direct reclaims are recorded using the + allocstall counter in /proc/vmstat. + + The range of acceptible values are 1-16. Always start with lower + values in the 2-6 range. Higher values should be justified with + testing. If direct reclaims occur in spite of high values, the cost + of direct reclaims (in latency) that occur can be higher due to + increased lock contention. + onenand.bdry= [HW,MTD] Flex-OneNAND Boundary Configuration Format: [die0_boundary][,die0_lock][,die1_boundary][,die1_lock] diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 1df06a7f10dd4..1ccd1ad16015c 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -38,6 +38,8 @@ */ #define PAGE_ALLOC_COSTLY_ORDER 3 +#define MAX_KSWAPD_THREADS 16 + enum migratetype { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, @@ -769,6 +771,7 @@ typedef struct pglist_data { wait_queue_head_t pfmemalloc_wait; struct task_struct *kswapd; /* Protected by mem_hotplug_begin/end() */ + struct task_struct *mkswapd[MAX_KSWAPD_THREADS]; int kswapd_order; enum zone_type kswapd_highest_zoneidx; diff --git a/mm/vmscan.c b/mm/vmscan.c index 4c5a9b2286bf5..0facbf9930d34 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -171,6 +171,23 @@ struct scan_control { */ int vm_swappiness = 60; +#define DEF_KSWAPD_THREADS_PER_NODE 1 +int kswapd_threads = DEF_KSWAPD_THREADS_PER_NODE; +static int __init kswapd_per_node_setup(char *str) +{ + int tmp; + + if (kstrtoint(str, 0, &tmp) < 0) + return 0; + + if (tmp > MAX_KSWAPD_THREADS || tmp <= 0) + return 0; + + kswapd_threads = tmp; + return 1; +} +__setup("kswapd_per_node=", kswapd_per_node_setup); + static void set_task_reclaim_state(struct task_struct *task, struct reclaim_state *rs) { @@ -3935,6 +3952,46 @@ static int kswapd(void *p) return 0; } +static int kswapd_per_node_run(int nid) +{ + pg_data_t *pgdat = NODE_DATA(nid); + int hid; + int ret = 0; + + for (hid = 0; hid < kswapd_threads; ++hid) { + pgdat->mkswapd[hid] = kthread_run(kswapd, pgdat, "kswapd%d:%d", + nid, hid); + if (IS_ERR(pgdat->mkswapd[hid])) { + /* failure at boot is fatal */ + WARN_ON(system_state < SYSTEM_RUNNING); + pr_err("Failed to start kswapd%d on node %d\n", + hid, nid); + ret = PTR_ERR(pgdat->mkswapd[hid]); + pgdat->mkswapd[hid] = NULL; + continue; + } + if (!pgdat->kswapd) + pgdat->kswapd = pgdat->mkswapd[hid]; + } + + return ret; +} + +static void kswapd_per_node_stop(int nid) +{ + int hid = 0; + struct task_struct *kswapd; + + for (hid = 0; hid < kswapd_threads; hid++) { + kswapd = NODE_DATA(nid)->mkswapd[hid]; + if (kswapd) { + kthread_stop(kswapd); + NODE_DATA(nid)->mkswapd[hid] = NULL; + } + } + NODE_DATA(nid)->kswapd = NULL; +} + /* * A zone is low on free memory or too fragmented for high-order memory. If * kswapd should reclaim (direct reclaim is deferred), wake it up for the zone's @@ -4038,6 +4095,9 @@ int kswapd_run(int nid) if (pgdat->kswapd) return 0; + if (kswapd_threads > 1) + return kswapd_per_node_run(nid); + pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid); if (IS_ERR(pgdat->kswapd)) { /* failure at boot is fatal */ @@ -4057,6 +4117,11 @@ void kswapd_stop(int nid) { struct task_struct *kswapd = NODE_DATA(nid)->kswapd; + if (kswapd_threads > 1) { + kswapd_per_node_stop(nid); + return; + } + if (kswapd) { kthread_stop(kswapd); NODE_DATA(nid)->kswapd = NULL;