linux-kernel - Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt

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Message-ID: <20191219140252.GS2871@hirez.programming.kicks-ass.net>
Date:   Thu, 19 Dec 2019 15:02:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in
 check_preempt_curr()

On Thu, Dec 19, 2019 at 02:12:42PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 19, 2019 at 03:39:14PM +0300, Kirill Tkhai wrote:
> > In kernel/sched/Makefile files, describing different sched classes, already
> > go in the order from the lowest priority class to the highest priority class:
> > 
> > idle.o fair.o rt.o deadline.o stop_task.o
> > 
> > The documentation of GNU linker says, that section appears in the order
> > they are seen during link time (see [1]):
> > 
> > >Normally, the linker will place files and sections matched by wildcards
> > >in the order in which they are seen during the link. You can change this
> > >by using the SORT keyword, which appears before a wildcard pattern
> > >in parentheses (e.g., SORT(.text*)).
> > 
> > So, we may expect const variables from idle.o will go before ro variables
> > from fair.o in RO_DATA section, while ro variables from fair.o will go
> > before ro variables from rt.o, etc.
> > 
> > (Also, it looks like the linking order is already used in kernel, e.g.
> >  in drivers/md/Makefile)
> > 
> > Thus, we may introduce an optimization based on xxx_sched_class addresses
> > in these two hot scheduler functions: pick_next_task() and check_preempt_curr().
> > 
> > One more result of the patch is that size of object file becomes a little
> > less (excluding added BUG_ON(), which goes in __init section):
> > 
> > $size kernel/sched/core.o
> >          text     data      bss	    dec	    hex	filename
> > before:  66446    18957	    676	  86079	  1503f	kernel/sched/core.o
> > after:   66398    18957	    676	  86031	  1500f	kernel/sched/core.o
> 
> Does LTO preserve this behaviour? I've never quite dared do this exact
> optimization.

Also, ld.lld seems a popular option.