Messages in this thread |  | | Subject | Re: [PATCH] scsi: fix sense_slab/bio swapping livelock | From | James Bottomley <> | Date | Sun, 06 Apr 2008 18:35:35 -0500 |
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On Sun, 2008-04-06 at 23:56 +0100, Hugh Dickins wrote: > Since 2.6.25-rc7, I've been seeing an occasional livelock on one > x86_64 machine, copying kernel trees to tmpfs, paging out to swap. > > Signature: 6000 pages under writeback but never getting written; > most tasks of interest trying to reclaim, but each get_swap_bio > waiting for a bio in mempool_alloc's io_schedule_timeout(5*HZ); > every five seconds an atomic page allocation failure report from > kblockd failing to allocate a sense_buffer in __scsi_get_command. > > __scsi_get_command has a (one item) free_list to protect against > this, but rc1's [SCSI] use dynamically allocated sense buffer > de25deb18016f66dcdede165d07654559bb332bc upset that slightly. > When it fails to allocate from the separate sense_slab, instead > of giving up, it must fall back to the command free_list, which > is sure to have a sense_buffer attached. > > Either my earlier -rc testing missed this, or there's some recent > contributory factor. One very significant factor is SLUB, which > merges slab caches when it can, and on 64-bit happens to merge > both bio cache and sense_slab cache into kmalloc's 128-byte cache: > so that under this swapping load, bios above are liable to gobble > up all the slots needed for scsi_cmnd sense_buffers below. > > That's disturbing behaviour, and I tried a few things to fix it. > Adding a no-op constructor to the sense_slab inhibits SLUB from > merging it, and stops all the allocation failures I was seeing; > but it's rather a hack, and perhaps in different configurations > we have other caches on the swapout path which are ill-merged. > > Another alternative is to revert the separate sense_slab, using > cache-line-aligned sense_buffer allocated beyond scsi_cmnd from > the one kmem_cache; but that might waste more memory, and is > only a way of diverting around the known problem. > > While I don't like seeing the allocation failures, and hate the > idea of all those bios piled up above a scsi host working one by > one, it does seem to emerge fairly soon with the livelock fix. > So lacking better ideas, stick with that one clear fix for now. > > Signed-off-by: Hugh Dickins <hugh@veritas.com>
This was sort of accidentally fixed in scsi-misc by commit
commit c5f73260b289cb974928eac05f2d84e58ddfc020 Author: James Bottomley <James.Bottomley@HansenPartnership.com> Date: Thu Mar 13 11:16:33 2008 -0500
[SCSI] consolidate command allocation in a single place
Could you check that:
master.kernel.org:/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6.git
and see if it alleviates the problem? ... if so, we can work out which pieces to backport.
Thanks,
James
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