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 9962A14E2F1; Sat, 3 Aug 2024 10:59:26 +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=1722682766; cv=none; b=EbSyBb8r7OA57zJ/2qm9804Ub/eur78j+WafgpoE6ctnV0Ge39/S5vj9ilno9O5vG481HbLz51QwGePvxMRi9kzmMc3ha1NnhcbGOpSIgh3HzliYw6gbpojwmzcGZx7yADPqfKE9CpW22//1T2gXzMSH5hP5lqX2WY3G+1e+CHM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722682766; c=relaxed/simple; bh=wXJibRRZdKerw8MBnL8BT75mXojPWhhoJOnbQymsNj0=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=e1aq4+TGaMymri9s+vxKZDNZc4hRswKkIIevR6e0VsZ6+2ea1LPpoeMIoWjoVCpSkMnkWIzM81kloFg5kvlSLECsPY+uGu403/n4SS/Jo6NQ9EGfBxiFYJK6ip6Zgu7G/20zjGmIpcGaMMLZfneSBEzHUiGKuw22m/A7/tmqqoI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=sg358cku; 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="sg358cku" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 68A2BC4AF0A; Sat, 3 Aug 2024 10:59:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1722682766; bh=wXJibRRZdKerw8MBnL8BT75mXojPWhhoJOnbQymsNj0=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=sg358ckufK+k2XFwJr6mNo76SJseGXXq+48T6xjlyWPDxe9Cz/ZO9vamfQ2nUfrMt Fa8HOF+JgsVF67w5hmesBkcIVUbgNUWbcu5awi/QMMCdo/YhjABsW7vrr/FmJ5RYn+ muClUkE+muPBsKVfEpjoxbz1UuKeI3e4W8gKZr69hV4ypvCsuS76s1vZa64T2IJ06b eF+qW8DiqvW0XemCxqy4luQjv6FzzBqSK9gBg8cNqHwwNjzldeZnuvyuMSdaeHF8bN 51/nG8FVmF5Sc7DvIo1sQgSUwaIQ+96ale2TSn2XZNKiOkrOSk4ZYAnqInQl3LYPrN raArjW4e/c3Gw== Message-ID: <7ff040d4a0fb1634d3dc9282da014165a347dbb2.camel@kernel.org> Subject: Re: [PATCH RFC 3/4] lockref: rework CMPXCHG_LOOP to handle contention better From: Jeff Layton To: Mateusz Guzik Cc: Alexander Viro , Christian Brauner , Jan Kara , Andrew Morton , Josef Bacik , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Date: Sat, 03 Aug 2024 06:59:24 -0400 In-Reply-To: References: <20240802-openfast-v1-0-a1cff2a33063@kernel.org> <20240802-openfast-v1-3-a1cff2a33063@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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1.0 On Sat, 2024-08-03 at 11:09 +0200, Mateusz Guzik wrote: > On Sat, Aug 3, 2024 at 6:44=E2=80=AFAM Mateusz Guzik = wrote: > >=20 > > On Fri, Aug 02, 2024 at 05:45:04PM -0400, Jeff Layton wrote: > > > In a later patch, we want to change the open(..., O_CREAT) codepath t= o > > > avoid taking the inode->i_rwsem for write when the dentry already exi= sts. > > > When we tested that initially, the performance devolved significantly > > > due to contention for the parent's d_lockref spinlock. > > >=20 > > > There are two problems with lockrefs today: First, once any concurren= t > > > task takes the spinlock, they all end up taking the spinlock, which i= s > > > much more costly than a single cmpxchg operation. The second problem = is > > > that once any task fails to cmpxchg 100 times, it falls back to the > > > spinlock. The upshot there is that even moderate contention can cause= a > > > fallback to serialized spinlocking, which worsens performance. > > >=20 > > > This patch changes CMPXCHG_LOOP in 2 ways: > > >=20 > > > First, change the loop to spin instead of falling back to a locked > > > codepath when the spinlock is held. Once the lock is released, allow = the > > > task to continue trying its cmpxchg loop as before instead of taking = the > > > lock. Second, don't allow the cmpxchg loop to give up after 100 retri= es. > > > Just continue infinitely. > > >=20 > > > This greatly reduces contention on the lockref when there are large > > > numbers of concurrent increments and decrements occurring. > > >=20 > >=20 > > This was already tried by me and it unfortunately can reduce performanc= e. > >=20 >=20 > Oh wait I misread the patch based on what I tried there. Spinning > indefinitely waiting for the lock to be free is a no-go as it loses > the forward progress guarantee (and it is possible to get the lock > being continuously held). Only spinning up to an arbitrary point wins > some in some tests and loses in others. >=20 I'm a little confused about the forward progress guarantee here. Does that exist today at all? ISTM that falling back to spin_lock() after a certain number of retries doesn't guarantee any forward progress. You can still just end up spinning on the lock forever once that happens, no? > Either way, as described below, chances are decent that: > 1. there is an easy way to not lockref_get/put on the parent if the > file is already there, dodging the problem > .. and even if that's not true > 2. lockref can be ditched in favor of atomics. apart from some minor > refactoring this all looks perfectly doable and I have a wip. I will > try to find the time next week to sort it out >=20 Like I said in the earlier mail, I don't think we can stay in RCU mode because of the audit_inode call. I'm definitely interested in your WIP though! > > Key problem is that in some corner cases the lock can be continuously > > held and be queued on, making the fast path always fail and making all > > the spins actively waste time (and notably pull on the cacheline). > >=20 > > See this for more details: > > https://lore.kernel.org/oe-lkp/lv7ykdnn2nrci3orajf7ev64afxqdw2d65bcpu2m= faqbkvv4ke@hzxat7utjnvx/ > >=20 > > However, I *suspect* in the case you are optimizing here (open + O_CREA= T > > of an existing file) lockref on the parent can be avoided altogether > > with some hackery and that's what should be done here. > >=20 > > When it comes to lockref in vfs in general, most uses can be elided wit= h > > some hackery (see the above thread) which is in early WIP (the LSMs are > > a massive headache). > >=20 > > For open calls which *do* need to take a real ref the hackery does not > > help of course. > >=20 > > This is where I think decoupling ref from the lock is the best way > > forward. For that to work the dentry must hang around after the last > > unref (already done thanks to RCU and dput even explicitly handles that > > already!) and there needs to be a way to block new refs atomically -- > > can be done with cmpxchg from a 0-ref state to a flag blocking new refs > > coming in. I have that as a WIP as well. > >=20 > >=20 > > > Signed-off-by: Jeff Layton > > > --- > > > lib/lockref.c | 85 ++++++++++++++++++++++---------------------------= ---------- > > > 1 file changed, 32 insertions(+), 53 deletions(-) > > >=20 > > > diff --git a/lib/lockref.c b/lib/lockref.c > > > index 2afe4c5d8919..b76941043fe9 100644 > > > --- a/lib/lockref.c > > > +++ b/lib/lockref.c > > > @@ -8,22 +8,25 @@ > > > * Note that the "cmpxchg()" reloads the "old" value for the > > > * failure case. > > > */ > > > -#define CMPXCHG_LOOP(CODE, SUCCESS) do { = \ > > > - int retry =3D 100; = \ > > > - struct lockref old; = \ > > > - BUILD_BUG_ON(sizeof(old) !=3D 8); = \ > > > - old.lock_count =3D READ_ONCE(lockref->lock_count); = \ > > > - while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock)= )) { \ > > > - struct lockref new =3D old; = \ > > > - CODE = \ > > > - if (likely(try_cmpxchg64_relaxed(&lockref->lock_count, = \ > > > - &old.lock_count, = \ > > > - new.lock_count))) { = \ > > > - SUCCESS; = \ > > > - } = \ > > > - if (!--retry) = \ > > > - break; = \ > > > - } = \ > > > +#define CMPXCHG_LOOP(CODE, SUCCESS) do { = \ > > > + struct lockref old; = \ > > > + BUILD_BUG_ON(sizeof(old) !=3D 8); = \ > > > + old.lock_count =3D READ_ONCE(lockref->lock_count); = \ > > > + for (;;) { = \ > > > + struct lockref new =3D old; = \ > > > + = \ > > > + if (likely(arch_spin_value_unlocked(old.lock.rlock.raw_= lock))) { \ > > > + CODE = \ > > > + if (likely(try_cmpxchg64_relaxed(&lockref->lock= _count, \ > > > + &old.lock_coun= t, \ > > > + new.lock_count= ))) { \ > > > + SUCCESS; = \ > > > + } = \ > > > + } else { = \ > > > + cpu_relax(); = \ > > > + old.lock_count =3D READ_ONCE(lockref->lock_coun= t); \ > > > + } = \ > > > + } = \ > > > } while (0) > > >=20 > > > #else > > > @@ -46,10 +49,8 @@ void lockref_get(struct lockref *lockref) > > > , > > > return; > > > ); > > > - > > > - spin_lock(&lockref->lock); > > > - lockref->count++; > > > - spin_unlock(&lockref->lock); > > > + /* should never get here */ > > > + WARN_ON_ONCE(1); > > > } > > > EXPORT_SYMBOL(lockref_get); > > >=20 > > > @@ -60,8 +61,6 @@ EXPORT_SYMBOL(lockref_get); > > > */ > > > int lockref_get_not_zero(struct lockref *lockref) > > > { > > > - int retval; > > > - > > > CMPXCHG_LOOP( > > > new.count++; > > > if (old.count <=3D 0) > > > @@ -69,15 +68,9 @@ int lockref_get_not_zero(struct lockref *lockref) > > > , > > > return 1; > > > ); > > > - > > > - spin_lock(&lockref->lock); > > > - retval =3D 0; > > > - if (lockref->count > 0) { > > > - lockref->count++; > > > - retval =3D 1; > > > - } > > > - spin_unlock(&lockref->lock); > > > - return retval; > > > + /* should never get here */ > > > + WARN_ON_ONCE(1); > > > + return -1; > > > } > > > EXPORT_SYMBOL(lockref_get_not_zero); > > >=20 > > > @@ -88,8 +81,6 @@ EXPORT_SYMBOL(lockref_get_not_zero); > > > */ > > > int lockref_put_not_zero(struct lockref *lockref) > > > { > > > - int retval; > > > - > > > CMPXCHG_LOOP( > > > new.count--; > > > if (old.count <=3D 1) > > > @@ -97,15 +88,9 @@ int lockref_put_not_zero(struct lockref *lockref) > > > , > > > return 1; > > > ); > > > - > > > - spin_lock(&lockref->lock); > > > - retval =3D 0; > > > - if (lockref->count > 1) { > > > - lockref->count--; > > > - retval =3D 1; > > > - } > > > - spin_unlock(&lockref->lock); > > > - return retval; > > > + /* should never get here */ > > > + WARN_ON_ONCE(1); > > > + return -1; > > > } > > > EXPORT_SYMBOL(lockref_put_not_zero); > > >=20 > > > @@ -125,6 +110,8 @@ int lockref_put_return(struct lockref *lockref) > > > , > > > return new.count; > > > ); > > > + /* should never get here */ > > > + WARN_ON_ONCE(1); > > > return -1; > > > } > > > EXPORT_SYMBOL(lockref_put_return); > > > @@ -171,8 +158,6 @@ EXPORT_SYMBOL(lockref_mark_dead); > > > */ > > > int lockref_get_not_dead(struct lockref *lockref) > > > { > > > - int retval; > > > - > > > CMPXCHG_LOOP( > > > new.count++; > > > if (old.count < 0) > > > @@ -180,14 +165,8 @@ int lockref_get_not_dead(struct lockref *lockref= ) > > > , > > > return 1; > > > ); > > > - > > > - spin_lock(&lockref->lock); > > > - retval =3D 0; > > > - if (lockref->count >=3D 0) { > > > - lockref->count++; > > > - retval =3D 1; > > > - } > > > - spin_unlock(&lockref->lock); > > > - return retval; > > > + /* should never get here */ > > > + WARN_ON_ONCE(1); > > > + return -1; > > > } > > > EXPORT_SYMBOL(lockref_get_not_dead); > > >=20 > > > -- > > > 2.45.2 > > >=20 >=20 >=20 >=20 --=20 Jeff Layton