linux-kernel - Re: [PATCH] sched: remove sched_find_first

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Message-ID: <20170514180917.bil7xr4bqtfk6fzj@gmail.com>
Date:   Sun, 14 May 2017 20:09:17 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Akinobu Mita <mita@...aclelinux.com>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched: remove sched_find_first_bit()


* Yury Norov <ynorov@...iumnetworks.com> wrote:

> sched_find_first_bit() is in fact the unrolled version of
> find_first_bit(), which is theoretically faster in some cases.
> But in the kernel it is called only in couple places in 
> kernel/sched/rt.c, and both of them are not looking like hot
> paths [...]

They are in terms of scheduling: pick_next_rt_entity() is in the RT scheduling 
fastpath.

Which makes me just suspicious of how careful this patch really is:

> that will doubtly achieve measurable benefit from using unrolled version of 
> find_first_bit() - there's no hard loops, and the execution path is not really 
> short.

... that's really just handwaving. Numbers please.

Thanks,

	Ingo