From: Dave Chinner <david@fromorbit.com>
To: Beata Michalska <b.michalska@samsung.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-api@vger.kernel.org, greg@kroah.com, jack@suse.cz,
tytso@mit.edu, adilger.kernel@dilger.ca, hughd@google.com,
lczerner@redhat.com, hch@infradead.org,
linux-ext4@vger.kernel.org, linux-mm@kvack.org,
kyungmin.park@samsung.com, kmpark@infradead.org
Subject: Re: [RFC v3 1/4] fs: Add generic file system event notifications
Date: Fri, 19 Jun 2015 10:03:41 +1000 [thread overview]
Message-ID: <20150619000341.GM10224@dastard> (raw)
In-Reply-To: <55828064.5040301@samsung.com>
On Thu, Jun 18, 2015 at 10:25:08AM +0200, Beata Michalska wrote:
> On 06/18/2015 01:06 AM, Dave Chinner wrote:
> > On Tue, Jun 16, 2015 at 03:09:30PM +0200, Beata Michalska wrote:
> >> Introduce configurable generic interface for file
> >> system-wide event notifications, to provide file
> >> systems with a common way of reporting any potential
> >> issues as they emerge.
> >>
> >> The notifications are to be issued through generic
> >> netlink interface by newly introduced multicast group.
> >>
> >> Threshold notifications have been included, allowing
> >> triggering an event whenever the amount of free space drops
> >> below a certain level - or levels to be more precise as two
> >> of them are being supported: the lower and the upper range.
> >> The notifications work both ways: once the threshold level
> >> has been reached, an event shall be generated whenever
> >> the number of available blocks goes up again re-activating
> >> the threshold.
> >>
> >> The interface has been exposed through a vfs. Once mounted,
> >> it serves as an entry point for the set-up where one can
> >> register for particular file system events.
> >>
> >> Signed-off-by: Beata Michalska <b.michalska@samsung.com>
> >
> > This has massive scalability problems:
....
> > Have you noticed that the filesystems have percpu counters for
> > tracking global space usage? There's good reason for that - taking a
> > spinlock in such a hot accounting path causes severe contention.
....
> > Then puts the entire netlink send path inside this spinlock, which
> > includes memory allocation and all sorts of non-filesystem code
> > paths. And it may be inside critical filesystem locks as well....
> >
> > Apart from the serialisation problem of the locking, adding
> > memory allocation and the network send path to filesystem code
> > that is effectively considered "innermost" filesystem code is going
> > to have all sorts of problems for various filesystems. In the XFS
> > case, we simply cannot execute this sort of function in the places
> > where we update global space accounting.
> >
> > As it is, I think the basic concept of separate tracking of free
> > space if fundamentally flawed. What I think needs to be done is that
> > filesystems need access to the thresholds for events, and then the
> > filesystems call fs_event_send_thresh() themselves from appropriate
> > contexts (ie. without compromising locking, scalability, memory
> > allocation recursion constraints, etc).
> >
> > e.g. instead of tracking every change in free space, a filesystem
> > might execute this once every few seconds from a workqueue:
> >
> > event = fs_event_need_space_warning(sb, <fs_free_space>)
> > if (event)
> > fs_event_send_thresh(sb, event);
> >
> > User still gets warnings about space usage, but there's no runtime
> > overhead or problems with lock/memory allocation contexts, etc.
>
> Having fs to keep a firm hand on thresholds limits would indeed be
> far more sane approach though that would require each fs to
> add support for that and handle most of it on their own. Avoiding
>> this was the main rationale behind this rfc.
> If fs people agree to that, I'll be more than willing to drop this
> in favour of the per-fs tracking solution.
> Personally, I hope they will.
I was hoping that you'd think a little more about my suggestion and
work out how to do background threshold event detection generically.
I kind of left it as "an exercise for the reader" because it seems
obvious to me.
Hint: ->statfs allows you to get the total, free and used space
from filesystems in a generic manner.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
WARNING: multiple messages have this Message-ID (diff)
From: Dave Chinner <david@fromorbit.com>
To: Beata Michalska <b.michalska@samsung.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-api@vger.kernel.org, greg@kroah.com, jack@suse.cz,
tytso@mit.edu, adilger.kernel@dilger.ca, hughd@google.com,
lczerner@redhat.com, hch@infradead.org,
linux-ext4@vger.kernel.org, linux-mm@kvack.org,
kyungmin.park@samsung.com, kmpark@infradead.org
Subject: Re: [RFC v3 1/4] fs: Add generic file system event notifications
Date: Fri, 19 Jun 2015 10:03:41 +1000 [thread overview]
Message-ID: <20150619000341.GM10224@dastard> (raw)
In-Reply-To: <55828064.5040301@samsung.com>
On Thu, Jun 18, 2015 at 10:25:08AM +0200, Beata Michalska wrote:
> On 06/18/2015 01:06 AM, Dave Chinner wrote:
> > On Tue, Jun 16, 2015 at 03:09:30PM +0200, Beata Michalska wrote:
> >> Introduce configurable generic interface for file
> >> system-wide event notifications, to provide file
> >> systems with a common way of reporting any potential
> >> issues as they emerge.
> >>
> >> The notifications are to be issued through generic
> >> netlink interface by newly introduced multicast group.
> >>
> >> Threshold notifications have been included, allowing
> >> triggering an event whenever the amount of free space drops
> >> below a certain level - or levels to be more precise as two
> >> of them are being supported: the lower and the upper range.
> >> The notifications work both ways: once the threshold level
> >> has been reached, an event shall be generated whenever
> >> the number of available blocks goes up again re-activating
> >> the threshold.
> >>
> >> The interface has been exposed through a vfs. Once mounted,
> >> it serves as an entry point for the set-up where one can
> >> register for particular file system events.
> >>
> >> Signed-off-by: Beata Michalska <b.michalska@samsung.com>
> >
> > This has massive scalability problems:
....
> > Have you noticed that the filesystems have percpu counters for
> > tracking global space usage? There's good reason for that - taking a
> > spinlock in such a hot accounting path causes severe contention.
....
> > Then puts the entire netlink send path inside this spinlock, which
> > includes memory allocation and all sorts of non-filesystem code
> > paths. And it may be inside critical filesystem locks as well....
> >
> > Apart from the serialisation problem of the locking, adding
> > memory allocation and the network send path to filesystem code
> > that is effectively considered "innermost" filesystem code is going
> > to have all sorts of problems for various filesystems. In the XFS
> > case, we simply cannot execute this sort of function in the places
> > where we update global space accounting.
> >
> > As it is, I think the basic concept of separate tracking of free
> > space if fundamentally flawed. What I think needs to be done is that
> > filesystems need access to the thresholds for events, and then the
> > filesystems call fs_event_send_thresh() themselves from appropriate
> > contexts (ie. without compromising locking, scalability, memory
> > allocation recursion constraints, etc).
> >
> > e.g. instead of tracking every change in free space, a filesystem
> > might execute this once every few seconds from a workqueue:
> >
> > event = fs_event_need_space_warning(sb, <fs_free_space>)
> > if (event)
> > fs_event_send_thresh(sb, event);
> >
> > User still gets warnings about space usage, but there's no runtime
> > overhead or problems with lock/memory allocation contexts, etc.
>
> Having fs to keep a firm hand on thresholds limits would indeed be
> far more sane approach though that would require each fs to
> add support for that and handle most of it on their own. Avoiding
>> this was the main rationale behind this rfc.
> If fs people agree to that, I'll be more than willing to drop this
> in favour of the per-fs tracking solution.
> Personally, I hope they will.
I was hoping that you'd think a little more about my suggestion and
work out how to do background threshold event detection generically.
I kind of left it as "an exercise for the reader" because it seems
obvious to me.
Hint: ->statfs allows you to get the total, free and used space
from filesystems in a generic manner.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
--
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next prev parent reply other threads:[~2015-06-19 0:05 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-16 13:09 [RFC v3 0/4] fs: Add generic file system event notifications Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-06-16 13:09 ` [RFC v3 1/4] " Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-06-16 16:21 ` Al Viro
2015-06-16 16:21 ` Al Viro
2015-06-16 16:21 ` Al Viro
2015-06-17 9:22 ` Beata Michalska
2015-06-17 9:22 ` Beata Michalska
2015-06-17 23:06 ` Dave Chinner
2015-06-17 23:06 ` Dave Chinner
2015-06-18 8:25 ` Beata Michalska
2015-06-18 8:25 ` Beata Michalska
2015-06-19 0:03 ` Dave Chinner [this message]
2015-06-19 0:03 ` Dave Chinner
2015-06-19 17:28 ` Beata Michalska
2015-06-19 17:28 ` Beata Michalska
2015-06-19 17:28 ` Beata Michalska
2015-06-19 23:21 ` Dave Chinner
2015-06-19 23:21 ` Dave Chinner
2015-06-22 15:46 ` Beata Michalska
2015-06-22 15:46 ` Beata Michalska
2015-06-18 11:17 ` Kinglong Mee
2015-06-18 11:17 ` Kinglong Mee
2015-06-18 14:50 ` Beata Michalska
2015-06-18 14:50 ` Beata Michalska
2015-06-24 8:47 ` Dmitry Monakhov
2015-06-24 8:47 ` Dmitry Monakhov
2015-06-24 15:31 ` Beata Michalska
2015-06-24 15:31 ` Beata Michalska
2015-06-24 16:26 ` Steve French
2015-06-24 16:26 ` Steve French
2015-06-26 7:30 ` Beata Michalska
2015-06-26 7:30 ` Beata Michalska
2015-07-22 15:55 ` Bartlomiej Zolnierkiewicz
2015-07-22 15:55 ` Bartlomiej Zolnierkiewicz
2015-07-30 8:22 ` Beata Michalska
2015-07-30 8:22 ` Beata Michalska
2015-06-16 13:09 ` [RFC v3 2/4] ext4: Add helper function to mark group as corrupted Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-07-22 10:40 ` Bartlomiej Zolnierkiewicz
2015-07-22 10:40 ` Bartlomiej Zolnierkiewicz
2015-06-16 13:09 ` [RFC v3 3/4] ext4: Add support for generic FS events Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-06-17 6:15 ` Leon Romanovsky
2015-06-17 6:15 ` Leon Romanovsky
2015-06-17 9:25 ` Beata Michalska
2015-06-17 9:25 ` Beata Michalska
2015-06-16 13:09 ` [RFC v3 4/4] shmem: " Beata Michalska
2015-06-16 13:09 ` Beata Michalska
2015-06-17 6:08 ` Leon Romanovsky
2015-06-17 6:08 ` Leon Romanovsky
2015-06-17 9:23 ` Beata Michalska
2015-06-17 9:23 ` Beata Michalska
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