On 2011-04-12 15:40, Dave Chinner wrote: > On Tue, Apr 12, 2011 at 02:28:31PM +0200, Jens Axboe wrote: >> On 2011-04-12 14:22, Dave Chinner wrote: >>> On Tue, Apr 12, 2011 at 10:36:30AM +0200, Jens Axboe wrote: >>>> On 2011-04-12 03:12, hch@xxxxxxxxxxxxx wrote: >>>>> On Mon, Apr 11, 2011 at 02:48:45PM +0200, Jens Axboe wrote: >>>>> function calls. >>>>> - Why is having a plug in blk_flush_plug marked unlikely? Note that >>>>> unlikely is the static branch prediction hint to mark the case >>>>> extremly unlikely and is even used for hot/cold partitioning. But >>>>> when we call it we usually check beforehand if we actually have >>>>> plugs, so it's actually likely to happen. >>>> >>>> The existance and out-of-line is for the scheduler() hook. It should be >>>> an unlikely event to schedule with a plug held, normally the plug should >>>> have been explicitly unplugged before that happens. >>> >>> Though if it does, haven't you just added a significant amount of >>> depth to the worst case stack usage? I'm seeing this sort of thing >>> from io_schedule(): >>> >>> Depth Size Location (40 entries) >>> ----- ---- -------- >>> 0) 4256 16 mempool_alloc_slab+0x15/0x20 >>> 1) 4240 144 mempool_alloc+0x63/0x160 >>> 2) 4096 16 scsi_sg_alloc+0x4c/0x60 >>> 3) 4080 112 __sg_alloc_table+0x66/0x140 >>> 4) 3968 32 scsi_init_sgtable+0x33/0x90 >>> 5) 3936 48 scsi_init_io+0x31/0xc0 >>> 6) 3888 32 scsi_setup_fs_cmnd+0x79/0xe0 >>> 7) 3856 112 sd_prep_fn+0x150/0xa90 >>> 8) 3744 48 blk_peek_request+0x6a/0x1f0 >>> 9) 3696 96 scsi_request_fn+0x60/0x510 >>> 10) 3600 32 __blk_run_queue+0x57/0x100 >>> 11) 3568 80 flush_plug_list+0x133/0x1d0 >>> 12) 3488 32 __blk_flush_plug+0x24/0x50 >>> 13) 3456 32 io_schedule+0x79/0x80 >>> >>> (This is from a page fault on ext3 that is doing page cache >>> readahead and blocking on a locked buffer.) > > FYI, the next step in the allocation chain adds >900 bytes to that > stack: > > $ cat /sys/kernel/debug/tracing/stack_trace > Depth Size Location (47 entries) > ----- ---- -------- > 0) 5176 40 zone_statistics+0xad/0xc0 > 1) 5136 288 get_page_from_freelist+0x2cf/0x840 > 2) 4848 304 __alloc_pages_nodemask+0x121/0x930 > 3) 4544 48 kmem_getpages+0x62/0x160 > 4) 4496 96 cache_grow+0x308/0x330 > 5) 4400 80 cache_alloc_refill+0x21c/0x260 > 6) 4320 64 kmem_cache_alloc+0x1b7/0x1e0 > 7) 4256 16 mempool_alloc_slab+0x15/0x20 > 8) 4240 144 mempool_alloc+0x63/0x160 > 9) 4096 16 scsi_sg_alloc+0x4c/0x60 > 10) 4080 112 __sg_alloc_table+0x66/0x140 > 11) 3968 32 scsi_init_sgtable+0x33/0x90 > 12) 3936 48 scsi_init_io+0x31/0xc0 > 13) 3888 32 scsi_setup_fs_cmnd+0x79/0xe0 > 14) 3856 112 sd_prep_fn+0x150/0xa90 > 15) 3744 48 blk_peek_request+0x6a/0x1f0 > 16) 3696 96 scsi_request_fn+0x60/0x510 > 17) 3600 32 __blk_run_queue+0x57/0x100 > 18) 3568 80 flush_plug_list+0x133/0x1d0 > 19) 3488 32 __blk_flush_plug+0x24/0x50 > 20) 3456 32 io_schedule+0x79/0x80 > > That's close to 1800 bytes now, and that's not entering the reclaim > path. If i get one deeper than that, I'll be sure to post it. :) Do you have traces from 2.6.38, or are you just doing them now? The path you quote above should not go into reclaim, it's a GFP_ATOMIC allocation. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
