linux-kernel - Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg

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Date:   Tue, 29 Jun 2021 12:26:21 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Dan Schatzberg <schatzberg.dan@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg

On Mon, Jun 28, 2021 at 10:17:18AM -0400, Dan Schatzberg <schatzberg.dan@...il.com> wrote:
> Agreed that exporting int_active_memcg is an implementation detail,
> but would this prevent set_active_memcg from being inlined?

Non-inlining in the loop module doesn't seem like a big trouble. OTOH,
other callers may be more sensitive and would need to rely on inlining.
I can't currently think of a nice way to have both the exported and the
exlicitly inlined variant at once. It seems it's either API or perf
craft in the end but both are uncertain, so I guess the current approach
is fine in the end.

> Yes it is intentional. All requests (not just aio) go through the loop
> worker which grabs the blkcg reference in loop_queue_work() on
> construction. So I believe grabbing a reference per request is
> unnecessary.

Isn't there a window without the reference between loop_queue_rq and
loop_queue_work? I don't know, you seem to know better, so I'd suggest
dropping a comment line into the code explaining this.

Thanks,
Michal

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