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Message-ID: <2e2636ef-eb0f-067d-ef8b-a95e762dbf9f@intel.com>
Date: Mon, 4 Nov 2024 19:29:00 -0800
From: Fenghua Yu <fenghua.yu@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, Peter Newman <peternewman@...gle.com>,
"Chatre, Reinette" <reinette.chatre@...el.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
"x86@...nel.org" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, "Babu
Moger" <babu.moger@....com>, James Morse <james.morse@....com>, "Martin
Kletzander" <nert.pinx@...il.com>, Shaopeng Tan <tan.shaopeng@...itsu.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Eranian,
Stephane" <eranian@...gle.com>
Subject: Re: [PATCH 2/2] x86/resctrl: Don't workqueue local event counter
reads
Hi, Tony,
On 11/4/24 16:12, Luck, Tony wrote:
>> Whenever this function is called, the performance is degraded rather
>> than improved because extra get_cpu()/put_cpu() are called in the fast
>> path in the current patch.
>
> But get_cpu()/put_cpu() aren't high overhead. Maybe costs less that the
> cpumask_any_housekeeping() call that is avoided by Peter's patch.
Quote from Peter:
"AMD EPYC 7B12 64-Core Processor (250 mon groups)
Local Domain: 3.25M -> 1.22M (-62.5%)
Remote Domain: 7.91M -> 8.05M (+2.9%)
Intel(R) Xeon(R) Gold 6268CL CPU @ 2.80GHz (190 mon groups)
Local Domain: 2.98M -> 2.21M (-25.8%)
Remote Domain: 4.49M -> 4.62M (+3.1%)
Note that there is a small increase in overhead for remote domains,
which results from the introduction of a put_cpu() call to reenable
preemption after determining whether the fast path can be used."
As his data shows, if the fast path is not taken, the extra put_cpu()
itself costs +2.9% extra time on AMD machine and +3.1% extra time on
Intel machine.
And this ~3% overhead is on top of queued work, which is more expensive
than cpumask_any_housekeeping() IIUC.
>
> Note that if Peter's patch doesn't take its fast path because the calling
> CPU was on the wrong domain, then the subsequent code is going to
> do an IPI whichever of the if/else path is taken.
In this case, actually IPI is only taken in smp_call_function_any() and
smp_call_on_cpu() invokes a queued work instead of IPI.
My proposed change logically doesn't change Peter's fast path and
performance for nohz_full/smp_call_on_cpu() case. It just utilizes the
"built-in fast path already" inside smp_call_function_any() to save
extra get_cpu() and put_cpu(). Hopefully the saved extra get_cpu() and
put_cpu() can offset cost of cpumask_any_housekeeping().
From Peter's commit message, seems nohz_full case is not
called/measured a lot if any. If only one or a very few housekeeping
CPUs on a large system, the nohz_full case will be called frequently and
fast path will fail most of time and the extra get_cpu()/put_cpu()
around the fast path might impact more on both local and total domain.
Thanks.
-Fenghua
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