Re: [PATCH] vfs: Avoid IPI storm due to bh LRU invalidation

[Jan Kara]

On Mon 06-02-12 13:17:17, Andrew Morton wrote:
> On Mon, 6 Feb 2012 17:47:32 +0100
> Jan Kara <jack@xxxxxxx> wrote:
> 
> > On Mon 06-02-12 21:12:36, Srivatsa S. Bhat wrote:
> > > On 02/06/2012 07:25 PM, Jan Kara wrote:
> > > 
> > > > When discovery of lots of disks happen in parallel, we call
> > > > invalidate_bh_lrus() once for each disk from partitioning code resulting in a
> > > > storm of IPIs and causing a softlockup detection to fire (it takes several
> > > > *minutes* for a machine to execute all the invalidate_bh_lrus() calls).
> 
> Gad.  How many disks are we talking about here?
  I think something around hundred scsi disks in this case (number of
physical drives is actually lower but multipathing blows it up). I actually
saw machines with close to thousand scsi disks (yes, they had names like
sdabc ;).

> > > > Fix the issue by allowing only single invalidation to run using a mutex and let
> > > > waiters for mutex figure out whether someone invalidated LRUs for them while
> > > > they were waiting.
> > > > 
> > > > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > > > ---
> > > >  fs/buffer.c |   23 ++++++++++++++++++++++-
> > > >  1 files changed, 22 insertions(+), 1 deletions(-)
> > > > 
> > > >   I feel this is slightly hacky approach but it works. If someone has better
> > > > idea, please speak up.
> > > > 
> > > 
> > > 
> > > Something related that you might be interested in:
> > > https://lkml.org/lkml/2012/2/5/109
> > > 
> > > (This is part of Gilad's patchset that tries to reduce cross-CPU IPI
> > > interference.)
> >   Thanks for the pointer. I didn't know about it. As Hannes wrote, this
> > need not be enough for our use case as there might indeed be some bhs in
> > the LRU. But I'd be interested how well the patchset works anyway. Maybe it
> > would be enough because after all when we invalidate LRUs subsequent
> > callers will see them empty and not issue IPI? Hannes, can you give a try
> > to the patches?
> 
> If that doesn't work then an option to think about is to have a bool to
> disable the bh LRU code.  That would add a test-n-branch to
> __find_get_block(), which wouldn't kill us.  Arrange for the LRU code
> to be disabled during device probing.  Or just leave the LRU disabled
> until very late in boot, perhaps.
> 
> Also, I'm wondering why we call invalidate_bh_lrus() at all during
> partition reading.  Presumably it's where we're shooting down the
> blockdev pagecache (you didn't tell us and I'm too lazy to hunt it
> down).  But do we really need to drop the pagecache at
> whatever-this-callsite-is?
  block/genhd.c has in register_disk():
       ...
       bdev = bdget_disk(disk, 0);
       if (!bdev)
               goto exit;

       bdev->bd_invalidated = 1;
       err = blkdev_get(bdev, FMODE_READ, NULL);
       if (err < 0)
               goto exit;
       blkdev_put(bdev, FMODE_READ);
       ...
  And in blkdev_put() (actually __blkdev_put()) bd_openers drops to 0 so we
call kill_bdev() which calls invalidate_bh_lrus(). So yes, we are
unnecessarily eager to flush things there but I'm not sure if I see
a cleaner solution.

								Honza

-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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