Messages in this thread | | | Date | Fri, 09 Jul 2021 14:37:26 -0300 | From | Marcelo Tosatti <> | Subject | [patch 0/5] optionally perform deferred actions on return to userspace |
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Changelog:
-v2: fix !CONFIG_SMP breakage (kernel robot) switch option to generic "quiesce_on_exit_to_usermode"
Summary of what was discussed on -v1:
1) The additional hooks to performance sensitive callbacks in mm/vmstat.c are protected by a static key, therefore workloads which do not enable this should not be impacted.
2) People would prefer the prctl() interface, but as noted in the option documentation (patch 1), the code added by this patchset should be reused by the prctl() interface, and the isolcpus option can then be deprecated.
3) Nobody has any other bright ideas for ways to solve this that would make this patch series obsolete.
4) The isolcpus= interface should switch to a cpuset based interface.
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The logic to disable vmstat worker thread, when entering nohz full, does not cover all scenarios. For example, it is possible for the following to happen:
1) enter nohz_full, which calls refresh_cpu_vm_stats, syncing the stats. 2) app runs mlock, which increases counters for mlock'ed pages. 3) start -RT loop
Since refresh_cpu_vm_stats from nohz_full logic can happen _before_ the mlock, vmstat shepherd can restart vmstat worker thread on the CPU in question.
To fix this, optionally quiesce deferred actions when returning to userspace, controllable by a new "quiesce_on_exit_to_usermode" isolcpus flag (default off).
See individual patches for details.
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