linux-kernel - Re: [PATCH 7/8] cfq-iosched: fold cfq_find_alloc_queue() into cfq_get

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Message-ID: <x49twuhrlml.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 09 Jun 2015 10:40:02 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, vgoyal@...hat.com,
	avanzini.arianna@...il.com
Subject: Re: [PATCH 7/8] cfq-iosched: fold cfq_find_alloc_queue() into cfq_get_queue()

Tejun Heo <tj@...nel.org> writes:

> cfq_find_alloc_queue() checks whether a queue actually needs to be
> allocated, which is unnecessary as its sole caller, cfq_get_queue(),
> only calls it if so.  Also, the oom queue fallback logic is scattered
> between cfq_get_queue() and cfq_find_alloc_queue().  There really
> isn't much going on in the latter and things can be made simpler by
> folding it into cfq_get_queue().
>
> This patch collapses cfq_find_alloc_queue() into cfq_get_queue().  The
> change is fairly straight-forward with one exception - async_cfqq is
> now initialized to NULL and the "!is_sync" test in the last if
> conditional is replaced with "async_cfqq" test.  This is because gcc
> (5.1.1) gets confused for some reason and warns that async_cfqq may be
> used uninitialized otherwise.  Oh well, the code isn't necessarily
> worse this way.
>
> This patch doesn't cause any functional difference.

The resulting code (introduced by the last patch, I know) is not ideal:

        rcu_read_lock();
        cfqg = cfq_lookup_create_cfqg(cfqd, bio_blkcg(bio));
        if (!cfqg) {
                cfqq = &cfqd->oom_cfqq;
                goto out;
        }

        if (!is_sync) {
                if (!ioprio_valid(cic->ioprio)) {
                        struct task_struct *tsk = current;
                        ioprio = task_nice_ioprio(tsk);
                        ioprio_class = task_nice_ioclass(tsk);
                }
                async_cfqq = cfq_async_queue_prio(cfqd, ioprio_class,
                ioprio);
                cfqq = *async_cfqq;
                if (cfqq)
                        goto out;
        }

As you mentioned, we don't need to lookup the cfqg for the async queue.
What's more is we could fallback to the oom_cfqq even if we had an
existing async cfqq.  I'm guessing you structured the code this way to
make the error path cleaner.  I don't think it's a big deal, as it
should be a rare occurrence, so...

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
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