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smtp.kernel.org (Postfix) with ESMTPSA id 58E5DC4CECC; Thu, 21 Nov 2024 14:16:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1732198581; bh=jpRTGVKAEwCkyxmVL8MPJVCxOWSyQ5Jpjt2erQeCglE=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=teYXSNKxIyi6O6wd2h+LHvtdEYDtZwLIlqUt8YXWePyEECs0IaLvLSQlOdXoZb9Xp PezHAUIZ7tHwYZPHuQdb+zU2/hGlk3QjhcsqBxaMkDkJnpq+yXgpu7Cn5L7bmUjDxs igIX1HpdvKd+47U6ZI9KIJy6mhD2NsrbxgN47911G3IZyRbuvpvyu6PbwXMgZ/7bR3 wMAYcP2mzqi6VAvUF6ynWnR9XxvNuFR1CYxibCSbKPcPpwQud7Q3wblVPqIFV7UlhI s/NUYPx7/q62XqXTaS03BhkzApQDLrhGvSYizpt2jNo3PFuxX2Oh3vws+ChPkesIak C2EIbijWTn5tg== Message-ID: Subject: Re: [PATCH v3 0/3] symlink length caching From: Jeff Layton To: Mateusz Guzik , brauner@kernel.org Cc: viro@zeniv.linux.org.uk, jack@suse.cz, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, hughd@google.com, linux-ext4@vger.kernel.org, tytso@mit.edu, linux-mm@kvack.org Date: Thu, 21 Nov 2024 09:16:19 -0500 In-Reply-To: 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 8DUz3ron BK8UqI9WSFm//p1G6DjO6zOC2LYWIlcqhjH0UjGBQm2xNsEryvBEqdR5K91TPy+AFhGtNQ9af8R3yQVaRYkBCbSj2OsJkSZt7QccKfBJNaF4WH7Qbhl0pVMs8sHM346MPyAYz4PDH8Opl+xOQ/Q7pDQF0EhcjhDIUbL2fAZhOWT7aabqCbVIZCL63sOObGcZnMIrveNARWCHlPaoechIyfhFCQi3ZyX/DuAXXN1ysrW5POmDDCm2jzjR4TInHciUXIBd+MbqCgJHB21w= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2024-11-20 at 12:20 +0100, Mateusz Guzik wrote: > quote: > When utilized it dodges strlen() in vfs_readlink(), giving about 1.5% > speed up when issuing readlink on /initrd.img on ext4. >=20 > The size is stored in a union with i_devices, which is never looked at > unless the inode is for a device. >=20 > Benchmark code at the bottom. >=20 > ext4 and tmpfs are patched, other filesystems can also get there with > some more work. >=20 > Arguably the current get_link API should be patched to let the fs return > the size, but that's not a churn I'm interested into diving in. >=20 > On my v1 Jan remarked 1.5% is not a particularly high win questioning > whether doing this makes sense. I noted the value is only this small > because of other slowdowns. >=20 > To elaborate here are highlights while benching on Sapphire Rapids: > 1. putname using atomics (over 3.5% on my profile) >=20 > sounds like Al has plans to do something here(?), I'm not touching it if > it can be helped. the atomic definitely does not have to be there in the > common case. >=20 > 2. kmem_cache_alloc_noprof/kmem_cache_free (over 7% combined)=20 >=20 > They are both dog slow due to cmpxchg16b. A patchset was posted which > adds a different allocation/free fast path which should whack majority > of the problem, see: https://lore.kernel.org/linux-mm/20241112-slub-percp= u-caches-v1-0-ddc0bdc27e05@suse.cz/ >=20 > If this lands I'll definitely try to make the pathname allocs use it, > should drop about 5-6 percentage points on this sucker. >=20 > 3. __legitimize_mnt issues a full fence (again over 3%) >=20 > As far as avoiding the fence is concerned waiting on rcu grace period on > unmount should do the trick. However, I found there is a bunch > complexity there to sort out before doing this will be feasible (notably > there are multiple mounts freed in one go, this needs to be batched). > There may be other issues which I missed and which make this not worth > it, but the fence is definitely avoidable in principle and I would be > surprised if there was no way to sensibly get there. No ETA, for now I'm > just pointing this out. >=20 > There is also the idea to speculatively elide lockref, but when I tried > that last time I ran into significant LSM-related trouble. >=20 > All that aside there is also quite a bit of branching and func calling > which does not need to be there (example: make vfsuid/vfsgid, could be > combined into one routine etc.). >=20 > Ultimately there is single-threaded perf left on the table in various > spots. >=20 > v3: > - use a union instead of a dedicated field, used up with i_devices >=20 > v2: > - add a dedicated field, flag and a helper instead of using i_size > https://lore.kernel.org/linux-fsdevel/20241119094555.660666-1-mjguzik@gma= il.com/ >=20 > v1 can be found here: > https://lore.kernel.org/linux-fsdevel/20241118085357.494178-1-mjguzik@gma= il.com/ >=20 > benchmark: > plug into will-it-scale into tests/readlink1.c: >=20 > #include > #include > #include > #include > #include > #include > #include >=20 > char *testcase_description =3D "readlink /initrd.img"; >=20 > void testcase(unsigned long long *iterations, unsigned long nr) > { > char *tmplink =3D "/initrd.img"; > char buf[1024]; >=20 > while (1) { > int error =3D readlink(tmplink, buf, sizeof(buf)); > assert(error > 0); >=20 > (*iterations)++; > } > } >=20 > Mateusz Guzik (3): > vfs: support caching symlink lengths in inodes > ext4: use inode_set_cached_link() > tmpfs: use inode_set_cached_link() >=20 > fs/ext4/inode.c | 3 ++- > fs/ext4/namei.c | 4 +++- > fs/namei.c | 34 +++++++++++++++++++--------------- > fs/proc/namespaces.c | 2 +- > include/linux/fs.h | 15 +++++++++++++-- > mm/shmem.c | 6 ++++-- > security/apparmor/apparmorfs.c | 2 +- > 7 files changed, 43 insertions(+), 23 deletions(-) >=20 Nice work, Mateusz! Reviewed-by: Jeff Layton