linux-kernel - Re: [PATCH v5 00/12] fold per-CPU vmstats remotely

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Message-ID: <ZBBv/l6NshXFI8vW@tpad>
Date:   Tue, 14 Mar 2023 10:00:46 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v5 00/12] fold per-CPU vmstats remotely

On Tue, Mar 14, 2023 at 09:59:37AM -0300, Marcelo Tosatti wrote:
> > Why is that a problem? Out-of-sync stats shouldn't cause major problems.
> > Or can they?
> 
> Consider SCHED_FIFO task that is polling the network queue (say
> testpmd).
> 
> 	do {
> 	 	if (net_registers->state & DATA_AVAILABLE) {
> 			process_data)();
> 		}
> 	 } while (!stopped);
> 
> Since this task runs at SCHED_FIFO priority, kworker won't 
> be scheduled to run (therefore per-CPU vmstats won't be
> flushed to global vmstats). 
> 
> Or, if testpmd runs at SCHED_OTHER, then the work item to
> flush per-CPU vmstats causes
> 
> 	testpmd -> kworker
> 	kworker: flush per-CPU vmstats
> 	kworker -> testpmd
> 
> And this might cause undesired latencies to the packets being
> processed by the testpmd task.

This problem is unrelated to the kswapd problem, but both are addressed
by the patchset.