linux-kernel - Re: [PATCH] block: switch to atomic

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Message-ID: <dbe2b14c-5f0a-80dd-b661-d0a7c74ba230@kernel.dk>
Date:   Mon, 6 Dec 2021 09:32:06 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        keescook@...omium.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: switch to atomic_t for request references

On 12/6/21 1:31 AM, Peter Zijlstra wrote:
> On Sun, Dec 05, 2021 at 10:53:49PM -0800, Christoph Hellwig wrote:
>> On Fri, Dec 03, 2021 at 08:35:40AM -0700, Jens Axboe wrote:
>>> refcount_t is not as expensive as it used to be, but it's still more
>>> expensive than the io_uring method of using atomic_t and just checking
>>> for potential over/underflow.
>>>
>>> This borrows that same implementation, which in turn is based on the
>>> mm implementation from Linus.
>>
>> If refcount_t isn't good enough for a normal kernel fast path we have
>> a problem.  Can we discuss that with the maintainers instead of coming
>> up with our home grown schemes again?
> 
> Quite; and for something that pretends to be about performance, it also
> lacks any actual numbers to back that claim.

I can certainly generate that, it was already done for the two previous
similar conversions though.

> The proposed implementation also doesn't do nearly as much as the
> refcount_t one does.
> 
> Anyway refcount_t is just a single "lock xadd" and a few branches, where
> does it go wrong? Do you have perf output to compare between them?

I'll generate that.

-- 
Jens Axboe