Messages in this thread | | | Date | Mon, 5 Jul 2021 16:26:48 +0200 (CEST) | From | Christoph Lameter <> | Subject | Re: [patch 0/5] optionally sync per-CPU vmstats counter on return to userspace |
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On Fri, 2 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? > > Hum, i don't think its a good idea to use that route, because > entering or exiting nohz_full depends on a number of variable > outside of one's control (and additional variables might be > added in the future).
Then I do not see any need for this patch. Because after a certain time of inactivity (after the mlock) the system will enter nohz_full again. If userspace has no direct control over nohz_full and can only wait then it just has to do so.
> So preparing the system to function > while entering nohz_full at any location seems the sane thing to do. > > And that would be at return to userspace (since, if mlocked, after > that point there will be no more changes to propagate to vmstat > counters). > > Or am i missing something else you can think of ?
I assumed that the "enter nohz full" was an action by the user space app because I saw some earlier patches to introduce such functionality in the past.
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