linux-kernel - Re: [PATCH] sched/cputime: Mitigate performance regression in times()/clock

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Date:	Wed, 27 Jul 2016 13:41:22 +0200
From:	Giovanni Gherdovich <ggherdovich@...e.cz>
To:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	kbuild-all@...org, Mike Galbraith <mgalbraith@...e.de>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.com>,
	mgorman@...hsingularity.net
Subject: Re: [PATCH] sched/cputime: Mitigate performance regression in
 times()/clock_gettime()

On Tue, 2016-07-26 at 22:43 +0800, kbuild test robot wrote:
> Hi,
> 
> [auto build test ERROR on tip/sched/core]
> [also build test ERROR on v4.7 next-20160726]
> [if your patch is applied to the wrong git tree, please drop us a
> note to help improve the system]
> 
> url:    https://github.com/0day-ci/linux/commits/Giovanni-Gherdovich/
> sched-cputime-Mitigate-performance-regression-in-times
> -clock_gettime/20160726-221216
> config: mips-allyesconfig (attached as .config)
> compiler: mips-linux-gnu-gcc (Debian 5.4.0-6) 5.4.0 20160609
> reproduce:
>         wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tes
> ts.git/plain/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # save the attached .config to linux build tree
>         make.cross ARCH=mips 
> 
> All errors (new ones prefixed by >>):
> 
>    kernel/sched/core.c: In function 'task_sched_runtime':
> > > kernel/sched/core.c:3009:3: error: implicit declaration of
> > > function 'prefetch' [-Werror=implicit-function-declaration]
>       prefetch((&p->se)->cfs_rq->curr);
>       ^
>    cc1: some warnings being treated as errors
>    make[2]: *** [kernel/sched/core.o] Error 1
>    make[2]: Target '__build' not remade because of errors.
>    make[1]: *** [kernel/sched/] Error 2
> 
> vim +/prefetch +3009 kernel/sched/core.c
> 
>   3003		 * Must be ->curr _and_ ->on_rq.  If dequeued,
> we would
>   3004		 * project cycles that may never be accounted
> to this
>   3005		 * thread, breaking clock_gettime().
>   3006		 */
>   3007		if (task_current(rq, p) &&
> task_on_rq_queued(p)) {
>   3008	#if defined(CONFIG_FAIR_GROUP_SCHED)
> > 3009			prefetch((&p->se)->cfs_rq->curr);
>   3010			prefetch(&(&p->se)->cfs_rq->curr
> ->exec_start);
>   3011	#endif
>   3012			update_rq_clock(rq);
> 
> ---
> 0-DAY kernel test infrastructure                Open Source
> Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Cor
> poration


As the robot pointed out, I forgot the appropriate header file. On x86
it compiled as the declaration must have come via an indirect header
file inclusion.

I will wait a few days for more comments, then I'll send a v2
addressing the missing #include with the following change:

-- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fde4014..97c196d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -74,6 +74,7 @@
 #include <linux/context_tracking.h>
 #include <linux/compiler.h>
 #include <linux/frame.h>
+#include <linux/prefetch.h>
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
-- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 -- -- >8 

Regards,
Giovanni Gherdovich
SUSE Labs