linux-kernel - Re: [dm-devel] [PATCH] sched: make struct task

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Message-ID: <20190903071326.GV2369@hirez.programming.kicks-ass.net>
Date:   Tue, 3 Sep 2019 09:13:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>, mingo@...hat.com,
        aarcange@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        dm-devel@...hat.com, axboe@...nel.dk
Subject: Re: [dm-devel] [PATCH] sched: make struct task_struct::state 32-bit

On Mon, Sep 02, 2019 at 11:51:55PM -0700, Christoph Hellwig wrote:
> On Tue, Sep 03, 2019 at 12:05:58AM +0300, Alexey Dobriyan wrote:
> > 32-bit accesses are shorter than 64-bit accesses on x86_64.
> > Nothing uses 64-bitness of ->state.
> > 
> > Space savings are ~2KB on F30 kernel config.
> 
> I guess we'd save even more when moving from a volatile to
> WRITE_ONCE/READ_ONCE..

I doubt it; pretty much all accesses really should be using that.

Not saying we shouldn't maybe do that; but that's going to be massive
churn.