From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6FD021BC072; Thu, 10 Oct 2024 09:06:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728551211; cv=none; b=hRWtgqKpIoNeZr8CmUjvJoTYfHaG17uBltpqmWKLlu0Vak/+/jrBoPHv7XOfu2pPECEjmGVbo71/hjBsR2xYRUvll+z8Nlei3/FRNSTTjgk1RF1epkZRfqq0SBFkoWeN7oB+sFQ2EF1Pc2ATlYCLqSlVTzVyN3QTQZdjwg2S6ng= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728551211; c=relaxed/simple; bh=v4F8XpUl4hzFdHfzDjb5HLOD91LVHfyBbNZzZzRfUAU=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=tA4vyqVmnk6OO75vamtImUOjlBkAHz0AQs/MA5+EO9vkuSVGgMha+SsuYPxYMRqQSrQsfwPPi5gqOFWO7HvSO3Q8ih+qQPCK3nAiHDvCrhmFpAPHi8QQ67YH9mfUgprZ5n0l8dI4/oBN53vHMI9/UkdH5eS9QewwuvT4TujKXi4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R0xTcg5L; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="R0xTcg5L" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1F882C4CEC5; Thu, 10 Oct 2024 09:06:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728551211; bh=v4F8XpUl4hzFdHfzDjb5HLOD91LVHfyBbNZzZzRfUAU=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=R0xTcg5L2aCx/ttD6ENUD6O6y1Zfwx3aCFVpWkNrHVIazxdpJw5ZtkdwfKBhtURRr nKs+katFJtGY4y9BHsx6YE5YLYjJyfbk5XB0Ad2iiIxeARpwAHC7FA+lJZdmhUd74G Sv67APBSjpoytNZZVp828xYUo8/esQSrHxYAzGzMi1pqcAAmjQcVphmr4yIWk06iuh 0yZ9pbVU6fAogCQZzdc83nl7zeVrgLPZLlbhR4nNZl8VIRciqGgae2hONIXtep4um7 EfOB3Vby2EJIgRv2v2i0IPeo+hySRYJ1+p/AJ+HhbBPMXpH9jnWoITGvuAkWNODkV/ UtSHXuY5irBBg== From: Andreas Hindborg To: "Gary Guo" Cc: "Alice Ryhl" , "Greg KH" , "Boqun Feng" , "Miguel Ojeda" , "Alex Gaynor" , =?utf-8?Q?Bj=C3=B6rn?= Roy Baron , "Benno Lossin" , "Trevor Gross" , "Jens Axboe" , "Will Deacon" , "Peter Zijlstra" , "Mark Rutland" , , , Subject: Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount` In-Reply-To: <871q0onyhf.fsf@kernel.org> (Andreas Hindborg's message of "Thu, 10 Oct 2024 10:39:08 +0200") References: <20241004155247.2210469-1-gary@garyguo.net> <20241004155247.2210469-4-gary@garyguo.net> <2024100507-percolate-kinship-fc9a@gregkh> <87zfniop6i.fsf@kernel.org> <20241005142332.78b8b5d0.gary@garyguo.net> <87msjioax2.fsf@kernel.org> <871q0onyhf.fsf@kernel.org> Date: Thu, 10 Oct 2024 11:06:36 +0200 Message-ID: <87set4min7.fsf@kernel.org> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Andreas Hindborg writes: > Andreas Hindborg writes: > >> "Gary Guo" writes: >> >>> On Sat, 5 Oct 2024 13:59:44 +0200 >>> Alice Ryhl wrote: >>> >>>> On Sat, Oct 5, 2024 at 11:49=E2=80=AFAM Andreas Hindborg wrote: >>>> > >>>> > Hi Greg, >>>> > >>>> > "Greg KH" writes: >>>> > >>>> > > On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote: >>>> > >> There is an operation needed by `block::mq`, atomically decreasing >>>> > >> refcount from 2 to 0, which is not available through refcount.h, = so >>>> > >> I exposed `Refcount::as_atomic` which allows accessing the refcou= nt >>>> > >> directly. >>>> > > >>>> > > That's scary, and of course feels wrong on many levels, but: >>>> > > >>>> > > >>>> > >> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &A= Ref) { >>>> > >> /// C `struct request`. If the operation fails, `this` is re= turned in the >>>> > >> /// `Err` variant. >>>> > >> fn try_set_end(this: ARef) -> Result<*mut bindings::re= quest, ARef> { >>>> > >> - // We can race with `TagSet::tag_to_rq` >>>> > >> - if let Err(_old) =3D this.wrapper_ref().refcount().compa= re_exchange( >>>> > >> - 2, >>>> > >> - 0, >>>> > >> - Ordering::Relaxed, >>>> > >> - Ordering::Relaxed, >>>> > >> - ) { >>>> > >> + // To hand back the ownership, we need the current refco= unt to be 2. >>>> > >> + // Since we can race with `TagSet::tag_to_rq`, this need= s to atomically reduce >>>> > >> + // refcount to 0. `Refcount` does not provide a way to d= o this, so use the underlying >>>> > >> + // atomics directly. >>>> > >> + if this >>>> > >> + .wrapper_ref() >>>> > >> + .refcount() >>>> > >> + .as_atomic() >>>> > >> + .compare_exchange(2, 0, Ordering::Relaxed, Ordering:= :Relaxed) >>>> > >> + .is_err() >>>> > > >>>> > > Why not just call rust_helper_refcount_set()? Or is the issue tha= t you >>>> > > think you might not be 2 here? And if you HAVE to be 2, why that = magic >>>> > > value (i.e. why not just always be 1 and rely on normal >>>> > > increment/decrement?) >>>> > > >>>> > > I know some refcounts are odd in the kernel, but I don't see where= the >>>> > > block layer is caring about 2 as a refcount anywhere, what am I mi= ssing? >>>> > >>>> > It is in the documentation, rendered version available here [1]. Let= me >>>> > know if it is still unclear, then I guess we need to update the docs. >>>> > >>>> > Also, my session from Recipes has a little bit of discussion regardi= ng >>>> > this refcount and it's use [2]. >>>> > >>>> > Best regards, >>>> > Andreas >>>> > >>>> > >>>> > [1] https://rust.docs.kernel.org/kernel/block/mq/struct.Request.html= #implementation-details >>>> > [2] https://youtu.be/1LEvgkhU-t4?si=3DB1XnJhzCCNnUtRsI&t=3D1685 >>>> >>>> So it sounds like there is one refcount from the C side, and some >>>> number of references from the Rust side. The function checks whether >>>> there's only one Rust reference left, and if so, takes ownership of >>>> the value, correct? >>>> >>>> In that case, the CAS should have an acquire ordering to synchronize >>>> with dropping the refcount 3->2 on another thread. Otherwise, you >>>> might have a data race with the operations that happened just before >>>> the 3->2 refcount drop. >>>> >>>> Alice >>> >>> The code as is is fine since there's no data protected in >>> `RequestDataWrapper` yet (in fact it's not even generic yet). I know >>> Andreas does want to introduce driver-specific data into that, so in >>> the long term the acquire would be necessary. >>> >>> Andreas, please let me know if you want me to make the change now, or >>> you'd rather change the ordering when you introduce data to >>> `RequestDataWrapper`. >> >> I guess we will have said data dependencies when we are going to run >> drop for fields in the private data area. Thanks for pointing that out. >> I will update the ordering when I submit that patch. >> >> As I mentioned before, I would rather we do not apply this patch before >> we get a way to inline helpers. > > As discussed offline, the code that suffers the performance regression > is downstream, and since this change seems to be important, I can apply > the helper LTO patch downstream as well. > > Since the plan for the downstream code _is_ to move upstream, I really > hope to see the helper LTO patch upstream, so we don't get a performance > regression because of these refcounts. > > If we cannot figure out a way to get the LTO patches (or an alternative > solution) upstream, we can always revert back to a more performant > solution in block. I forgot to report the result of the benchmarks. Over the usual benchmark workload that I run for `rnull` I see an average 0.8 percent performance penalty with this patch. For some configurations I see 95% CI N=3D40 [-18%;-5%]. So it is not insignificant. BR Andreas