Commits on Jan 17, 2024

1. scx_rustland: avoid calling scx_bpf_kick_cpu() from update_idle() …

   Prior to commit 676bd88 ("bpf_rustland: do not dispatch the scheduler to
   the global DSQ"), the user-space scheduler was dispatched using
   SCX_DSQ_GLOBAL and we needed to explicitly kick idle CPUs from
   update_idle() to ensure that at least one CPU was available to run the
   user-space scheduler.
   
   Now that we are using SCX_DSQ_LOCAL_ON|cpu to dispatch the user-space
   scheduler, the target CPU is implicitly kicked. Therefore, the call to
   scx_bpf_kick_cpu() within .update_idle() becomes redundant and we can
   get rid of it.
   
   Fixes: 676bd88 ("bpf_rustland: do not dispatch the scheduler to the global DSQ")
   Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

   

   arighi committed Jan 17, 2024
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2. scx_rustland: improve SMT performance …

   The user-space scheduler dispatches tasks in batches, with the batch
   size matching the number of idle CPUs.
   
   Commit 791bdbe ("scx_rustland: introduce SMT support") changed the order
   of idle CPUs, prioritizing dispatching tasks on the least busy cores
   (those with the most idle CPUs) before moving on to busier cores (those
   with the least idle CPUs).
   
   While this approach works well for a small number of tasks, it can lead
   to uneven performance as the number of tasks increases and all cores are
   saturated. Such uneven performance can be attributed to SMT interactions
   causing potential short lags and erratic system performance. In some
   cases, disabling SMT entirely results in better system responsiveness.
   
   To address this issue, instruct the scheduler to implicitly disable SMT
   and consistently dispatch tasks only on the first (or last) CPU of each
   core. This approach ensures an equal distribution of tasks among the
   available cores, preventing SMT disturbances and aligning with non-SMT
   performance, also when a significant amount of tasks are running.
   
   Additionally, the unused sibling CPUs within each core can be used as
   "spare" CPUs for the BPF dispatcher. This is particularly beneficial for
   tasks that cannot be dispatched on the target CPU selected by the
   scheduler, due to cpumask restrictions or congestion conditions.
   
   Therefore, this new approach allows to enhance system responsiveness on
   SMT systems, while simultaneously improving scheduler stability.
   
   Some preliminary results on an AMD Ryzen 7 5800X 8-Cores (SMT enabled):
   running my usual benchmark of measuring the fps of a videogame
   (Counter-Strike 2) during a parallel kernel build-induced system
   overload, shows an improvement of approximately 2x (from 8-10fps to
   15-25fps vs 1-2fps with EEVDF).
   
   Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

   

   arighi committed Jan 17, 2024
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