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Date:   Tue, 5 Jun 2018 13:45:56 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Josef Bacik <josef@toxicpanda.com>
Cc:     axboe@kernel.dk, kernel-team@fb.com, linux-block@vger.kernel.org,
        akpm@linux-foundation.org, hannes@cmpxchg.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Josef Bacik <jbacik@fb.com>
Subject: Re: [PATCH 06/13] blkcg: add generic throttling mechanism
Message-ID: <20180605204556.GJ1351649@devbig577.frc2.facebook.com>
References: <20180605132948.1664-1-josef@toxicpanda.com>
 <20180605132948.1664-7-josef@toxicpanda.com>
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On Tue, Jun 05, 2018 at 09:29:41AM -0400, Josef Bacik wrote:
> From: Josef Bacik <jbacik@fb.com>
> 
> Since IO can be issued from literally anywhere it's almost impossible to
> do throttling without having some sort of adverse effect somewhere else
> in the system because of locking or other dependencies.  The best way to
> solve this is to do the throttling when we know we aren't holding any
> other kernel resources.  Do this by tracking throttling in a per-blkg
> basis, and if we require throttling flag the task that it needs to check
> before it returns to user space and possibly sleep there.
> 
> This is to address the case where a process is doing work that is
> generating IO that can't be throttled, whether that is directly with a
> lot of REQ_META IO, or indirectly by allocating so much memory that it
> is swamping the disk with REQ_SWAP.  We can't use task_add_work as we
> don't want to induce a memory allocation in the IO path, so simply
> saving the request queue in the task and flagging it to do the
> notify_resume thing achieves the same result without the overhead of a
> memory allocation.
> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>

Acked-by: Tejun Heo <tj@kernel.org>

-- 
tejun