linux-kernel - Re: [PATCH 5/8] cfq-iosched: remove @gfp_mask from cfq_find_alloc

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Date:	Mon, 08 Jun 2015 15:24:12 -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 5/8] cfq-iosched: remove @gfp_mask from cfq_find_alloc_queue()

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

> Even when allocations fail, cfq_find_alloc_queue() always returns a
> valid cfq_queue by falling back to the oom cfq_queue.  As such, there
> isn't much point in taking @gfp_mask and trying "harder" if __GFP_WAIT
> is set.  GFP_ATOMIC allocations don't fail often and even when they do
> the degraded behavior is acceptable and temporary.
>
> After all, the only reason get_request(), which ultimately determines
> the gfp_mask, cares about __GFP_WAIT is to guarantee request
> allocation, assuming IO forward progress, for callers which are
> willing to wait.  There's no reason for cfq_find_alloc_queue() to
> behave differently on __GFP_WAIT when it already has a fallback
> mechanism.
>
> Remove @gfp_mask from cfq_find_alloc_queue() and propagate the changes
> to its callers.  This simplifies the function quite a bit and will
> help making async queues per-cfq_group.

Sorry, I disagree with this patch.  You've changed it so that all cfqq
allocations are GFP_ATOMIC, and most, if not all of them simply don't
need to be.

I'll take it one step further and suggest that we should fix
cfq_find_alloc_queue to pass the gfp_mask to check_ioprio_changed.  That
also shouldn't be using GFP_ATOMIC unconditionally.

NAK

-Jeff
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