1. 06 Apr, 2008 1 commit
    • Hugh Dickins's avatar
      scsi: fix sense_slab/bio swapping livelock · 164fc5dc
      Hugh Dickins authored
      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
      de25deb1 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: default avatarHugh Dickins <hugh@veritas.com>
      Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
      Cc: Andrew Morton <akpm@linux-foundation.org>
      Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
      Cc: Jens Axboe <jens.axboe@oracle.com>
      Cc: Christoph Lameter <clameter@sgi.com>
      Cc: Pekka Enberg <penberg@cs.helsinki.fi>
      Cc: Peter Zijlstra <a.p.ziljstra@chello.nl>
      Cc: Rafael J. Wysocki <rjw@sisk.pl>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      164fc5dc
  2. 05 Apr, 2008 1 commit
  3. 04 Apr, 2008 27 commits
  4. 03 Apr, 2008 11 commits