linux-kernel - Re: [PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat

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Message-ID: <ZpBIfwThiVwstlXL@slm.duckdns.org>
Date: Thu, 11 Jul 2024 11:02:55 -1000
From: "tj@...nel.org" <tj@...nel.org>
To: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>
Cc: "boris@....io" <boris@....io>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat_lock for
 stats update

Hello,

On Thu, Jul 11, 2024 at 02:25:29AM +0000, Boy Wu (吳勃誼) wrote:
...
> I can remove the 32bit only define, but I think we need to add back the
> u64 sync, because the spin lock and the u64 sync serve different
> purposes. The spin lock is for handling concurrent problems from
> different CPUs updating stats, and u64 sync is for updating 64 bits
> data and fetching 64 bits data from different CPUs in 32bit SMP
> systems.

Hmm... so what I'm suggesting is using u64_sync for the per-cpu stat
structure as they are guaranteed to have only one updater with irq disabled
and use a spinlock for the shared iostat which can have multiple updaters
and isn't accessed that frequently.

Thanks.

-- 
tejun