linux-kernel - Re: [PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a user->kernel transition

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Message-ID: <aO-vkAAk4FCdnLZu@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 15 Oct 2025 16:28:32 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, rcu@...r.kernel.org,
	x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
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	Daniel Wagner <dwagner@...e.de>, Petr Tesarik <ptesarik@...e.com>
Subject: Re: [PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a
 user->kernel transition

On 15/10/25 15:16, Valentin Schneider wrote:
> On 14/10/25 17:26, Valentin Schneider wrote:
> > On 14/10/25 14:58, Juri Lelli wrote:
> >>> Noise
> >>> +++++
> >>>
> >>> Xeon E5-2699 system with SMToff, NOHZ_FULL, isolated CPUs.
> >>> RHEL10 userspace.
> >>>
> >>> Workload is using rteval (kernel compilation + hackbench) on housekeeping CPUs
> >>> and a dummy stay-in-userspace loop on the isolated CPUs. The main invocation is:
> >>>
> >>> $ trace-cmd record -e "ipi_send_cpumask" -f "cpumask & CPUS{$ISOL_CPUS}" \
> >>>                 -e "ipi_send_cpu"     -f "cpu & CPUS{$ISOL_CPUS}" \
> >>>                 rteval --onlyload --loads-cpulist=$HK_CPUS \
> >>>                 --hackbench-runlowmem=True --duration=$DURATION
> >>>
> >>> This only records IPIs sent to isolated CPUs, so any event there is interference
> >>> (with a bit of fuzz at the start/end of the workload when spawning the
> >>> processes). All tests were done with a duration of 6 hours.
> >>>
> >>> v6.17
> >>> o ~5400 IPIs received, so about ~200 interfering IPI per isolated CPU
> >>> o About one interfering IPI just shy of every 2 minutes
> >>>
> >>> v6.17 + patches
> >>> o Zilch!
> >>
> >> Nice. :)
> >>
> >> About performance, can we assume housekeeping CPUs are not affected by
> >> the change (they don't seem to use the trick anyway) or do we want/need
> >> to collect some numbers on them as well just in case (maybe more
> >> throughput oriented)?
> >>
> >
> > So for the text_poke IPI yes, because this is all done through
> > context_tracking which doesn't imply housekeeping CPUs.
> >
> > For the TLB flush faff the HK CPUs get two extra writes per kernel entry
> > cycle (one at entry and one at exit, for that stupid signal) which I expect
> > to be noticeable but small-ish. I can definitely go and measure that.
> >
> 
> On that same Xeon E5-2699 system with the same tuning, the average time
> taken for 300M gettid syscalls on housekeeping CPUs is
>   v6.17:          698.64ns ± 2.35ns
>   v6.17 + series: 702.60ns ± 3.43ns
> 
> So noticeable (~.6% worse) but not horrible?

Yeah, seems reasonable.

Thanks for collecting numbers!