On Fri, May 30, 2014 at 08:06:53PM -0600, Jens Axboe wrote: > On 2014-05-28 20:42, Linus Torvalds wrote: > >Well, we've definitely have had some issues with deeper callchains > >with md, but I suspect virtio might be worse, and the new blk-mq code > >is lilkely worse in this respect too. > > I don't think blk-mq is worse than the older stack, in fact it > should be better. The call chains are shorter, and a lot less cruft > on the stack. Historically the stack issues have been nested > devices, however. And for sync IO, we do run it inline, so if the > driver chews up a lot of stack, well... Hi Jens - as we found out with the mm code, there's a significant disconnect between what the code looks like (i.e. it may use very little stack directly) and what the compiler is generating. Before blk-mq: 9) 3952 112 scsi_request_fn+0x4b/0x490 10) 3840 32 __blk_run_queue+0x37/0x50 11) 3808 64 queue_unplugged+0x39/0xb0 12) 3744 112 blk_flush_plug_list+0x20b/0x240 Now with blk-mq: 3) 4672 96 virtio_queue_rq+0xd2/0x1e0 4) 4576 128 __blk_mq_run_hw_queue+0x1f0/0x3e0 5) 4448 16 blk_mq_run_hw_queue+0x35/0x40 6) 4432 80 blk_mq_insert_requests+0xc7/0x130 7) 4352 96 blk_mq_flush_plug_list+0x129/0x140 8) 4256 112 blk_flush_plug_list+0xe7/0x230 So previously flushing a plug used rough 200 bytes of stack. With blk-mq, it's over 400 bytes. IOWs, blk-mq has more than doubled the block layer stack usage... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>