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 DE62C1A8C05 for ; Thu, 20 Jun 2024 10:18:53 +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=1718878733; cv=none; b=ARHV8wzAPK22+lNkkxDg8UwyUYAvh9O/iW3B8fYFYa8Wld8YGlWxJZ/ZHJORPJmoRaSE9xO3y6eGofYqFBVKqUD6i9nAiXJP8JUX9V5B4pELe3SPY6ChnY5BWQc7739ftB+yWgGZJ1DfGyD82R1PsedMSCU1Cjr5gU1NialttRU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718878733; c=relaxed/simple; bh=E4/Z4NDMVUyAKOu0T40w/+rOpxPsOMauE03Si9z+VA8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=BHCi2Xai9jwCGzYnOppnQbMAy0laIkXe8mO5sEbQs3HjE4PydOQFwwf/fcaJUUf4LopgIFKapZdrvI+t5i1Q/4NEgN56ofllfOl1i/QontRQtRdQGEZNNuX0Iu7ATbyK18NfnYVjB7I/O068tow3GfDW6QW9uU+zMOzv3cnmiNI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dF7PGGyB; 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="dF7PGGyB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB472C2BD10; Thu, 20 Jun 2024 10:18:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1718878733; bh=E4/Z4NDMVUyAKOu0T40w/+rOpxPsOMauE03Si9z+VA8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=dF7PGGyBpVtDV3a72UdBhyn2MxSVRZyUT4sdq9EPp16vPDGwtH9Fakl76hIRmdfDc Qi7b7qHwTwMSDRup3w7CK6+gL+Y4y0kBeeINkQ930iYufmzOXQR3JuX6kZ9v36XbKm yIW/5JEYDMEyElgF8ddMPipeYKbxx/j7ye8w8ei0Axz2wS3qqZQ4+Y7cA6NgDbn3+d Ze0dMbEKUeB0VKRheht/gTuXu9ZHPIEMTl7rmGCP/Lf0rRQ/5ypGL66JkhfPOiyep1 cSMSZAfU+sBaUrCgg0dCkkbFPK+rju7P1l9/7Ypx1dmq7+d6eriLIsVdQQZWsYx4n5 51zzhZ5JtNa9A== Message-ID: Subject: Re: knfsd performance From: Jeff Layton To: Dave Chinner , NeilBrown Cc: Chuck Lever III , Trond Myklebust , Linux NFS Mailing List Date: Thu, 20 Jun 2024 06:18:51 -0400 In-Reply-To: References: <> <171875886281.14261.15016610844409785952@noble.neil.brown.name> <171883231568.14261.16495433738354176501@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV 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> > > On Wed, 19 Jun 2024, Dave Chinner wrote: > > > > On Tue, Jun 18, 2024 at 07:54:43PM +0000, Chuck Lever III wrote > = On Jun 18, 2024, at 3:50=E2=80=AFPM, Trond Myklebust wrote: > > > > > >=20 > > > > > > On Tue, 2024-06-18 at 19:39 +0000, Chuck Lever III wrote: > > > > > > >=20 > > > > > > >=20 > > > > > > > > On Jun 18, 2024, at 3:29=E2=80=AFPM, Trond Myklebust > > > > > > > > wrote: > > > > > > > >=20 > > > > > > > > On Tue, 2024-06-18 at 18:40 +0000, Chuck Lever III wrote: > > > > > > > > >=20 > > > > > > > > >=20 > > > > > > > > > > On Jun 18, 2024, at 2:32=E2=80=AFPM, Trond Myklebust > > > > > > > > > > wrote: > > > > > > > > > >=20 > > > > > > > > > > I recently back ported Neil's lwq code and sunrpc serve= r > > > > > > > > > > changes to > > > > > > > > > > our > > > > > > > > > > 5.15.130 based kernel in the hope of improving the perf= ormance > > > > > > > > > > for > > > > > > > > > > our > > > > > > > > > > data servers. > > > > > > > > > >=20 > > > > > > > > > > Our performance team recently ran a fio workload on a c= lient > > > > > > > > > > that > > > > > > > > > > was > > > > > > > > > > doing 100% NFSv3 reads in O_DIRECT mode over an RDMA co= nnection > > > > > > > > > > (infiniband) against that resulting server. I've attach= ed the > > > > > > > > > > resulting > > > > > > > > > > flame graph from a perf profile run on the server side. > > > > > > > > > >=20 > > > > > > > > > > Is anyone else seeing this massive contention for the s= pin lock > > > > > > > > > > in > > > > > > > > > > __lwq_dequeue? As you can see, it appears to be dwarfin= g all > > > > > > > > > > the > > > > > > > > > > other > > > > > > > > > > nfsd activity on the system in question here, being res= ponsible > > > > > > > > > > for > > > > > > > > > > 45% > > > > > > > > > > of all the perf hits. > > > >=20 > > > > Ouch. __lwq_dequeue() runs llist_reverse_order() under a spinlock. > > > >=20 > > > > llist_reverse_order() is an O(n) algorithm involving full length > > > > linked list traversal. IOWs, it's a worst case cache miss algorithm > > > > running under a spin lock. And then consider what happens when > > > > enqueue processing is faster than dequeue processing. > > >=20 > > > My expectation was that if enqueue processing (incoming packets) was > > > faster than dequeue processing (handling NFS requests) then there was= a > > > bottleneck elsewhere, and this one wouldn't be relevant. > > >=20 > > > It might be useful to measure how long the queue gets. > >=20 > > Thinking about this some more .... if it did turn out that the queue > > gets long, and maybe even if it didn't, we could reimplement lwq as a > > simple linked list with head and tail pointers. > >=20 > > enqueue would be something like: > >=20 > > new->next =3D NULL; > > old_tail =3D xchg(&q->tail, new); > > if (old_tail) > > /* dequeue of old_tail cannot succeed until this assignment comp= letes */ > > old_tail->next =3D new > > else > > q->head =3D new > >=20 > > dequeue would be > >=20 > > spinlock() > > ret =3D q->head; > > if (ret) { > > while (ret->next =3D=3D NULL && cmp_xchg(&q->tail, ret, NULL) != =3D ret) > > /* wait for enqueue of q->tail to complete */ > > cpu_relax(); > > } > > cmp_xchg(&q->head, ret, ret->next); > > spin_unlock(); >=20 > That might work, but I suspect that it's still only putting off the > inevitable. >=20 > Doing the dequeue purely with atomic operations might be possible, > but it's not immediately obvious to me how to solve both head/tail > race conditions with atomic operations. I can work out an algorithm > that makes enqueue safe against dequeue races (or vice versa), but I > can't also get the logic on the opposite side to also be safe. >=20 > I'll let it bounce around my head a bit more... >=20 The latest version of the multigrain timestamp patches doesn't use it, but Jan Kara pointed out to me that there is a cmpxchg128 function in the kernel. It's only defined for some arches (x86, s390, and aarch64, I think), but maybe that could be used here. --=20 Jeff Layton