From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755891AbYDFWyS (ORCPT ); Sun, 6 Apr 2008 18:54:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754090AbYDFWyD (ORCPT ); Sun, 6 Apr 2008 18:54:03 -0400 Received: from extu-mxob-1.symantec.com ([216.10.194.28]:46495 "EHLO extu-mxob-1.symantec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753927AbYDFWyC (ORCPT ); Sun, 6 Apr 2008 18:54:02 -0400 Date: Sun, 6 Apr 2008 23:56:57 +0100 (BST) From: Hugh Dickins X-X-Sender: hugh@blonde.site To: James Bottomley cc: Linus Torvalds , Andrew Morton , FUJITA Tomonori , Jens Axboe , Christoph Lameter , Pekka Enberg , Peter Zijlstra , "Rafael J. Wysocki" , linux-kernel@vger.kernel.org Subject: [PATCH] scsi: fix sense_slab/bio swapping livelock Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- drivers/scsi/scsi.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) --- 2.6.25-rc8/drivers/scsi/scsi.c 2008-03-05 10:47:40.000000000 +0000 +++ linux/drivers/scsi/scsi.c 2008-04-05 22:23:40.000000000 +0100 @@ -181,6 +181,18 @@ struct scsi_cmnd *__scsi_get_command(str cmd = kmem_cache_alloc(shost->cmd_pool->cmd_slab, gfp_mask | shost->cmd_pool->gfp_mask); + if (likely(cmd)) { + buf = kmem_cache_alloc(shost->cmd_pool->sense_slab, + gfp_mask | shost->cmd_pool->gfp_mask); + if (likely(buf)) { + memset(cmd, 0, sizeof(*cmd)); + cmd->sense_buffer = buf; + } else { + kmem_cache_free(shost->cmd_pool->cmd_slab, cmd); + cmd = NULL; + } + } + if (unlikely(!cmd)) { unsigned long flags; @@ -197,16 +209,6 @@ struct scsi_cmnd *__scsi_get_command(str memset(cmd, 0, sizeof(*cmd)); cmd->sense_buffer = buf; } - } else { - buf = kmem_cache_alloc(shost->cmd_pool->sense_slab, - gfp_mask | shost->cmd_pool->gfp_mask); - if (likely(buf)) { - memset(cmd, 0, sizeof(*cmd)); - cmd->sense_buffer = buf; - } else { - kmem_cache_free(shost->cmd_pool->cmd_slab, cmd); - cmd = NULL; - } } return cmd;