Messages in this thread | | | Date | Fri, 2 Jul 2021 08:52:35 -0300 | From | Marcelo Tosatti <> | Subject | Re: [patch 0/5] optionally sync per-CPU vmstats counter on return to userspace |
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Hi Christoph,
On Fri, Jul 02, 2021 at 10:00:11AM +0200, Christoph Lameter wrote: > On Thu, 1 Jul 2021, Marcelo Tosatti wrote: > > > 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. > > Can we enter nohz_full after the app runs mlock? > > > To fix this, optionally sync the vmstat counters when returning > > from userspace, controllable by a new "vmstat_sync" isolcpus > > flags (default off). > > > > See individual patches for details. > > Wow... This is going into some performance sensitive VM counters here and > adds code to their primitives.
Yes, but it should all be under static key (therefore the performance impact, when isolcpus=vmstat_sync,CPULIST is not enabled, should be zero) (if the patchset is correct! ...).
For the case where isolcpus=vmstat_sync is enabled, the most important performance aspect is the latency spike which this patch is dealing with.
> Isnt there a simpler solution that does not require this amount of > changes?
The one other change (I can think of) which could solve this problem would be allowing remote access to per-CPU vmstat counters (requiring a local_lock to be added), which seems to be more complex than this.
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