linux-kernel - Re: [PATCH 3/3] blk-throttle: don't parse cgroup path if trace isn't enabled

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Date:	Wed, 11 May 2016 09:54:40 -0700
From:	Shaohua Li <shli@...com>
To:	Simon Guo <wei.guo.simon@...il.com>
CC:	<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>, <axboe@...com>,
	<Kernel-team@...com>
Subject: Re: [PATCH 3/3] blk-throttle: don't parse cgroup path if trace isn't
 enabled

On Tue, May 10, 2016 at 01:46:13PM +0800, Simon Guo wrote:
> Hi, 
> On Mon, May 09, 2016 at 05:22:15PM -0700, Shaohua Li wrote:
> > if trace isn't enabled, parsing cgroup path just wastes cpu
> > 
> > Signed-off-by: Shaohua Li <shli@...com>
> > ---
> >  block/blk-throttle.c         | 5 ++---
> >  include/linux/blktrace_api.h | 9 +++++++++
> >  2 files changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > index 2149a1d..47a3e54 100644
> > --- a/block/blk-throttle.c
> > +++ b/block/blk-throttle.c
> > @@ -211,15 +211,14 @@ static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
> >   *
> >   * The messages are prefixed with "throtl BLKG_NAME" if @sq belongs to a
> >   * throtl_grp; otherwise, just "throtl".
> > - *
> > - * TODO: this should be made a function and name formatting should happen
> > - * after testing whether blktrace is enabled.
> >   */
> >  #define throtl_log(sq, fmt, args...)	do {				\
> >  	struct throtl_grp *__tg = sq_to_tg((sq));			\
> >  	struct throtl_data *__td = sq_to_td((sq));			\
> >  									\
> >  	(void)__td;							\
> > +	if (likely(!blk_trace_note_message_enabled(__td->queue)))	\
> > +		break;							\
> >  	if ((__tg)) {							\
> >  		char __pbuf[128];					\
> >  									\
> > diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
> > index afc1343..0f3172b 100644
> > --- a/include/linux/blktrace_api.h
> > +++ b/include/linux/blktrace_api.h
> > @@ -57,6 +57,14 @@ void __trace_note_message(struct blk_trace *, const char *fmt, ...);
> >  	} while (0)
> >  #define BLK_TN_MAX_MSG		128
> >  
> > +static inline bool blk_trace_note_message_enabled(struct request_queue *q)
> > +{
> > +	struct blk_trace *bt = q->blk_trace;
> > +	if (likely(!bt))
> > +		return false;
> > +	return bt->act_mask & BLK_TC_NOTIFY;
> > +}
> Is there any reason to skip following condition?
>         if (unlikely(bt->trace_state != Blktrace_running &&
>                      !blk_tracer_enabled))

It's marked unlikely and to add the check we must export
blk_tracer_enabled, so I ignored it. Though I don't know how unlikely it
is.

Thanks,
Shaohua