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Fri, 12 Jan 2024 06:11:44 -0800 (PST) Received: from [192.168.1.150] ([198.8.77.194]) by smtp.gmail.com with ESMTPSA id v3-20020aa78083000000b006d9b4303f9csm3218949pff.71.2024.01.12.06.11.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Jan 2024 06:11:43 -0800 (PST) Message-ID: <4a35c78e-98d4-4edb-b7bf-8a6d1df3c554@kernel.dk> Date: Fri, 12 Jan 2024 07:11:42 -0700 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC][PATCH v2] fsnotify: optimize the case of no content event watchers Content-Language: en-US From: Jens Axboe To: Amir Goldstein , Jan Kara Cc: Christian Brauner , linux-fsdevel@vger.kernel.org References: <20240111152233.352912-1-amir73il@gmail.com> <20240112110936.ibz4s42x75mjzhlv@quack3> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/12/24 6:58 AM, Jens Axboe wrote: > On 1/12/24 6:00 AM, Amir Goldstein wrote: >> On Fri, Jan 12, 2024 at 1:09?PM Jan Kara wrote: >>> >>> On Thu 11-01-24 17:22:33, Amir Goldstein wrote: >>>> Commit e43de7f0862b ("fsnotify: optimize the case of no marks of any type") >>>> optimized the case where there are no fsnotify watchers on any of the >>>> filesystem's objects. >>>> >>>> It is quite common for a system to have a single local filesystem and >>>> it is quite common for the system to have some inotify watches on some >>>> config files or directories, so the optimization of no marks at all is >>>> often not in effect. >>>> >>>> Content event (i.e. access,modify) watchers on sb/mount more rare, so >>>> optimizing the case of no sb/mount marks with content events can improve >>>> performance for more systems, especially for performance sensitive io >>>> workloads. >>>> >>>> Set a per-sb flag SB_I_CONTENT_WATCHED if sb/mount marks with content >>>> events in their mask exist and use that flag to optimize out the call to >>>> __fsnotify_parent() and fsnotify() in fsnotify access/modify hooks. >>>> >>>> Signed-off-by: Amir Goldstein >>> >>> ... >>> >>>> -static inline int fsnotify_file(struct file *file, __u32 mask) >>>> +static inline int fsnotify_path(const struct path *path, __u32 mask) >>>> { >>>> - const struct path *path; >>>> + struct dentry *dentry = path->dentry; >>>> >>>> - if (file->f_mode & FMODE_NONOTIFY) >>>> + if (!fsnotify_sb_has_watchers(dentry->d_sb)) >>>> return 0; >>>> >>>> - path = &file->f_path; >>>> + /* Optimize the likely case of sb/mount/parent not watching content */ >>>> + if (mask & FSNOTIFY_CONTENT_EVENTS && >>>> + likely(!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED)) && >>>> + likely(!(dentry->d_sb->s_iflags & SB_I_CONTENT_WATCHED))) { >>>> + /* >>>> + * XXX: if SB_I_CONTENT_WATCHED is not set, checking for content >>>> + * events in s_fsnotify_mask is redundant, but it will be needed >>>> + * if we use the flag FS_MNT_CONTENT_WATCHED to indicate the >>>> + * existence of only mount content event watchers. >>>> + */ >>>> + __u32 marks_mask = d_inode(dentry)->i_fsnotify_mask | >>>> + dentry->d_sb->s_fsnotify_mask; >>>> + >>>> + if (!(mask & marks_mask)) >>>> + return 0; >>>> + } >>> >>> So I'm probably missing something but how is all this patch different from: >>> >>> if (likely(!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED))) { >>> __u32 marks_mask = d_inode(dentry)->i_fsnotify_mask | >>> path->mnt->mnt_fsnotify_mask | >> >> It's actually: >> >> real_mount(path->mnt)->mnt_fsnotify_mask >> >> and this requires including "internal/mount.h" in all the call sites. >> >>> dentry->d_sb->s_fsnotify_mask; >>> if (!(mask & marks_mask)) >>> return 0; >>> } >>> >>> I mean (mask & FSNOTIFY_CONTENT_EVENTS) is true for the frequent events >>> (read & write) we care about. In Jens' case >>> >>> !(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED) && >>> !(dentry->d_sb->s_iflags & SB_I_CONTENT_WATCHED) >>> >>> is true as otherwise we'd go right to fsnotify_parent() and so Jens >>> wouldn't see the performance benefit. But then with your patch you fetch >>> i_fsnotify_mask and s_fsnotify_mask anyway for the test so the only >>> difference to what I suggest above is the path->mnt->mnt_fsnotify_mask >>> fetch but that is equivalent to sb->s_iflags (or wherever we store that >>> bit) fetch? >>> >>> So that would confirm that the parent handling costs in fsnotify_parent() >>> is what's really making the difference and just avoiding that by checking >>> masks early should be enough? >> >> Can't the benefit be also related to saving a function call? >> >> Only one way to find out... >> >> Jens, >> >> Can you please test attached v3 with a non-inlined fsnotify_path() helper? > > I can run it since it doesn't take much to do that, but there's no way > parallel universe where saving a function call would yield those kinds > of wins (or have that much cost). Ran this patch, and it's better than mainline for sure, but it does have additional overhead that the previous version did not: +1.46% [kernel.vmlinux] [k] fsnotify_path -- Jens Axboe