linux-kernel - Re: [PATCH v2 00/11] fold per-CPU vmstats remotely

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Message-ID: <Y/0HXjDXtMSKlwR8@tpad>
Date:   Mon, 27 Feb 2023 16:41:18 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 00/11] fold per-CPU vmstats remotely

On Fri, Feb 24, 2023 at 10:34:10AM +0800, Hillf Danton wrote:
> On Thu, Feb 09, 2023 at 12:01:50PM -0300, Marcelo Tosatti wrote:
> > This patch series addresses the following two problems:
> > 
> >     1. A customer provided some evidence which indicates that
> >        the idle tick was stopped; albeit, CPU-specific vmstat
> >        counters still remained populated.
> > 
> >        Thus one can only assume quiet_vmstat() was not
> >        invoked on return to the idle loop. If I understand
> >        correctly, I suspect this divergence might erroneously
> >        prevent a reclaim attempt by kswapd. If the number of
> >        zone specific free pages are below their per-cpu drift
> >        value then zone_page_state_snapshot() is used to
> >        compute a more accurate view of the aforementioned
> >        statistic.  Thus any task blocked on the NUMA node
> >        specific pfmemalloc_wait queue will be unable to make
> >        significant progress via direct reclaim unless it is
> >        killed after being woken up by kswapd
> >        (see throttle_direct_reclaim())
> > 
> >     2. With a SCHED_FIFO task that busy loops on a given CPU,
> >        and kworker for that CPU at SCHED_OTHER priority,
> >        queuing work to sync per-vmstats will either cause that
> >        work to never execute, or stalld (i.e. stall daemon)
> >        boosts kworker priority which causes a latency
> >        violation
> > 
> > By having vmstat_shepherd flush the per-CPU counters to the
> > global counters from remote CPUs.
> > 
> > This is done using cmpxchg to manipulate the counters,
> > both CPU locally (via the account functions),
> > and remotely (via cpu_vm_stats_fold).
> 
> Frankly another case of bandaid[1] ?
> 
> [1] https://lore.kernel.org/lkml/20230223150624.GA29739@lst.de/

Only if you disable per-CPU vmstat counters for isolated CPUs
(then maintenance of the data structures in isolated CPUs is
not necessary).

Which would be terrible for performance, however.