netdev - Re: [PATCH 11/16] time: optimize tick_check

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Date:   Tue, 9 Aug 2022 14:29:12 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <yury.norov@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Ben Segall <bsegall@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dennis Zhou <dennis@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Ingo Molnar <mingo@...hat.com>,
        Isabella Basso <isabbasso@...eup.net>,
        John Fastabend <john.fastabend@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        KP Singh <kpsingh@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Mel Gorman <mgorman@...e.de>, Miroslav Benes <mbenes@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yonghong Song <yhs@...com>,
        linux-mm@...ck.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 11/16] time: optimize tick_check_preferred()

On 08/08/2022 18.38, Yury Norov wrote:
> On Mon, Aug 08, 2022 at 01:42:54PM +0200, Thomas Gleixner wrote:
>> On Sat, Aug 06 2022 at 10:30, Thomas Gleixner wrote:
>>> On Mon, Jul 18 2022 at 12:28, Yury Norov wrote:
>>>
>>>> tick_check_preferred() calls cpumask_equal() even if
>>>> curdev->cpumask == newdev->cpumask. Fix it.
>>>
>>> What's to fix here? It's a pointless operation in a slow path and all
>>> your "fix' is doing is to make the code larger.
> 
> Pointless operation in a slow path is still pointless.
>  
>> In fact cpumask_equal() should have the ptr1 == ptr2 check, so you don't
>> have to add it all over the place.
> 
> This adds to the image size:
> add/remove: 1/1 grow/shrink: 24/3 up/down: 507/-46 (461)
> 
> The more important, cpumask shouldn't check parameters because this is
> an internal function. This whole series point is about adding such checks
> under DEBUG_BITMAP config, and not affecting general case.

Yury, calling bitmap_equal (and by extension cpumask_equal) with
something that happens in some cases to be the same pointer for both
operands is not a bug.

If you want to optimize that case, add a check in __bitmap_equal(), it
will add a few bytes (maybe 2-4 instructions) to the kernel image, much
less than what this whole series does by open-coding that check all
over, and since it's comparing two registers, it won't in any way affect
the performance of the case where the pointers are distinct.

Rasmus