linux-kernel - Re: [PATCH RFC] blkcg: prepare blkcg knobs for default hierarchy

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Date:	Tue, 27 May 2014 08:49:12 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Li Zefan <lizefan@...wei.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC] blkcg: prepare blkcg knobs for default hierarchy

On Fri, May 23, 2014 at 01:39:57PM -0400, Tejun Heo wrote:
> Hello, Vivek.
> 
> On Wed, Apr 23, 2014 at 03:21:09PM -0400, Vivek Goyal wrote:
> > What about sync/async differentiation? Throttling layer seems to flag a request sync
> > only if bio->bi_rw flag has REQ_SYNC set. While CFQ seems to consider
> > request sync if bio is either read or bio->bi_rw has REQ_SYNC flag set.
> 
> Working on this again, AFAICS, both treat REQ_SYNC the same way as far
> as stats are concerned.  If SYNC is set, it's sync; otherwise, it's
> accounted as async whether read or write.

Ok, that seems to be the case.

static inline void blkg_rwstat_add(struct blkg_rwstat *rwstat,
                                   int rw, uint64_t val)
{
        u64_stats_update_begin(&rwstat->syncp);

        if (rw & REQ_SYNC)
                rwstat->cnt[BLKG_RWSTAT_SYNC] += val;
        else
                rwstat->cnt[BLKG_RWSTAT_ASYNC] += val;

        u64_stats_update_end(&rwstat->syncp);
}


So sync will represent not policy specific interpretation of sync but
based on sync flag on request.

I guess it is fine. So far nobody seems to be complaining.

Thanks
Vivek
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