Re: [RFC 2/2] x86_64: expand kernel stack to 16K

Dave Chinner
2014-05-30T00:16:04Z

On Fri, May 30, 2014 at 08:36:38AM +0900, Minchan Kim wrote:
> Hello Dave,
>
> On Thu, May 29, 2014 at 11:58:30AM +1000, Dave Chinner wrote:
> > On Thu, May 29, 2014 at 11:30:07AM +1000, Dave Chinner wrote:
> > > On Wed, May 28, 2014 at 03:41:11PM -0700, Linus Torvalds wrote:
> > > commit a237c1c5bc5dc5c76a21be922dca4826f3eca8ca
> > > Author: Jens Axboe <jaxboe@fusionio.com>
> > > Date: Sat Apr 16 13:27:55 2011 +0200
> > >
> > > block: let io_schedule() flush the plug inline
> > >
> > > Linus correctly observes that the most important dispatch cases
> > > are now done from kblockd, this isn't ideal for latency reasons.
> > > The original reason for switching dispatches out-of-line was to
> > > avoid too deep a stack, so by _only_ letting the "accidental"
> > > flush directly in schedule() be guarded by offload to kblockd,
> > > we should be able to get the best of both worlds.
> > >
> > > So add a blk_schedule_flush_plug() that offloads to kblockd,
> > > and only use that from the schedule() path.
> > >
> > > Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
> > >
> > > And now we have too deep a stack due to unplugging from io_schedule()...
> >
> > So, if we make io_schedule() push the plug list off to the kblockd
> > like is done for schedule()....
....
> I did below hacky test to apply your idea and the result is overflow again.
> So, again it would second stack expansion. Otherwise, we should prevent
> swapout in direct reclaim.
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f5c6635b806c..95f169e85dbe 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4241,10 +4241,13 @@ EXPORT_SYMBOL_GPL(yield_to);
> void __sched io_schedule(void)
> {
> struct rq *rq = raw_rq();
> + struct blk_plug *plug = current->plug;
>
> delayacct_blkio_start();
> atomic_inc(&rq->nr_iowait);
> - blk_flush_plug(current);
> + if (plug)
> + blk_flush_plug_list(plug, true);
> +
> current->in_iowait = 1;
> schedule();
> current->in_iowait = 0;

.....

> Depth Size Location (46 entries)
>
> 0) 7200 8 _raw_spin_lock_irqsave+0x51/0x60
> 1) 7192 296 get_page_from_freelist+0x886/0x920
> 2) 6896 352 __alloc_pages_nodemask+0x5e1/0xb20
> 3) 6544 8 alloc_pages_current+0x10f/0x1f0
> 4) 6536 168 new_slab+0x2c5/0x370
> 5) 6368 8 __slab_alloc+0x3a9/0x501
> 6) 6360 80 __kmalloc+0x1cb/0x200
> 7) 6280 376 vring_add_indirect+0x36/0x200
> 8) 5904 144 virtqueue_add_sgs+0x2e2/0x320
> 9) 5760 288 __virtblk_add_req+0xda/0x1b0
> 10) 5472 96 virtio_queue_rq+0xd3/0x1d0
> 11) 5376 128 __blk_mq_run_hw_queue+0x1ef/0x440
> 12) 5248 16 blk_mq_run_hw_queue+0x35/0x40
> 13) 5232 96 blk_mq_insert_requests+0xdb/0x160
> 14) 5136 112 blk_mq_flush_plug_list+0x12b/0x140
> 15) 5024 112 blk_flush_plug_list+0xc7/0x220
> 16) 4912 128 blk_mq_make_request+0x42a/0x600
> 17) 4784 48 generic_make_request+0xc0/0x100
> 18) 4736 112 submit_bio+0x86/0x160
> 19) 4624 160 __swap_writepage+0x198/0x230
> 20) 4464 32 swap_writepage+0x42/0x90
> 21) 4432 320 shrink_page_list+0x676/0xa80
> 22) 4112 208 shrink_inactive_list+0x262/0x4e0
> 23) 3904 304 shrink_lruvec+0x3e1/0x6a0

The device is supposed to be plugged here in shrink_lruvec().

Oh, a plug can only hold 16 individual bios, and then it does a
synchronous flush. Hmmm - perhaps that should also defer the flush
to the kblockd, because if we are overrunning a plug then we've
already surrendered IO dispatch latency....

So, in blk_mq_make_request(), can you do:

if (list_empty(&plug->mq_list))
trace_block_plug(q);
else if (request_count >= BLK_MAX_REQUEST_COUNT) {
- blk_flush_plug_list(plug, false);
+ blk_flush_plug_list(plug, true);
trace_block_plug(q);
}
list_add_tail(&rq->queuelist, &plug->mq_list);

To see if that defers all the swap IO to kblockd?

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com