Date:   Mon, 20 Apr 2020 09:49:43 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Paolo Valente <paolo.valente@...aro.org>,
        Guenter Roeck <groeck@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>,
        linux-scsi@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>, Salman Qazi <sqazi@...gle.com>,
        André Almeida <andrealmeid@...labora.com>,
        Bart Van Assche <bvanassche@....org>,
        Damien Le Moal <damien.lemoal@....com>,
        John Garry <john.garry@...wei.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        LKML <linux-kernel@...r.kernel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>
Subject: Re: [PATCH v4 0/4] blk-mq: Fix two causes of IO stalls found in
 reboot testing

On 4/20/20 8:45 AM, Doug Anderson wrote:
> Hi Jens,
> 
> On Wed, Apr 8, 2020 at 8:35 AM Douglas Anderson <dianders@...omium.org> wrote:
>>
>> While doing reboot testing, I found that occasionally my device would
>> trigger the hung task detector.  Many tasks were stuck waiting for the
>> a blkdev mutex, but at least one task in the system was always sitting
>> waiting for IO to complete (and holding the blkdev mutex).  One
>> example of a task that was just waiting for its IO to complete on one
>> reboot:
>>
>>  udevd           D    0  2177    306 0x00400209
>>  Call trace:
>>   __switch_to+0x15c/0x17c
>>   __schedule+0x6e0/0x928
>>   schedule+0x8c/0xbc
>>   schedule_timeout+0x9c/0xfc
>>   io_schedule_timeout+0x24/0x48
>>   do_wait_for_common+0xd0/0x160
>>   wait_for_completion_io_timeout+0x54/0x74
>>   blk_execute_rq+0x9c/0xd8
>>   __scsi_execute+0x104/0x198
>>   scsi_test_unit_ready+0xa0/0x154
>>   sd_check_events+0xb4/0x164
>>   disk_check_events+0x58/0x154
>>   disk_clear_events+0x74/0x110
>>   check_disk_change+0x28/0x6c
>>   sd_open+0x5c/0x130
>>   __blkdev_get+0x20c/0x3d4
>>   blkdev_get+0x74/0x170
>>   blkdev_open+0x94/0xa8
>>   do_dentry_open+0x268/0x3a0
>>   vfs_open+0x34/0x40
>>   path_openat+0x39c/0xdf4
>>   do_filp_open+0x90/0x10c
>>   do_sys_open+0x150/0x3c8
>>   ...
>>
>> I've reproduced this on two systems: one boots from an internal UFS
>> disk and one from eMMC.  Each has a card reader attached via USB with
>> an SD card plugged in.  On the USB-attached SD card is a disk with 12
>> partitions (a Chrome OS test image), if it matters.  The system
>> doesn't do much with the USB disk other than probe it (it's plugged in
>> my system to help me recover).
>>
>> From digging, I believe that there are two separate but related
>> issues.  Both issues relate to the SCSI code saying that there is no
>> budget.
>>
>> I have done testing with only one or the other of the two patches in
>> this series and found that I could still encounter hung tasks if only
>> one of the two patches was applied.  This deserves a bit of
>> explanation.  To me, it's fairly obvious that the first fix wouldn't
>> fix the problems talked about in the second patch.  However, it's less
>> obvious why the second patch doesn't fix the problems in
>> blk_mq_dispatch_rq_list().  It turns out that it _almost_ does
>> (problems become much more rare), but I did manage to get a single
>> trace where the "kick" scheduled by the second patch happened really
>> quickly.  The scheduled kick then ran and found nothing to do.  This
>> happened in parallel to a task running in blk_mq_dispatch_rq_list()
>> which hadn't gotten around to splicing the list back into
>> hctx->dispatch.  This is why we need both fixes.
>>
>> Most of my testing has been atop Chrome OS 5.4's kernel tree which
>> currently has v5.4.30 merged in.  The Chrome OS 5.4 tree also has a
>> patch by Salman Qazi, namely ("block: Limit number of items taken from
>> the I/O scheduler in one go").  Reverting that patch didn't make the
>> hung tasks go away, so I kept it in for most of my testing.
>>
>> I have also done some testing on mainline Linux (most on what git
>> describe calls v5.6-rc7-227-gf3e69428b5e2) even without Salman's
>> patch.  I found that I could reproduce the problems there and that
>> traces looked about the same as I saw on the downstream branch.  These
>> patches were also confirmed to fix the problems on mainline.
>>
>> Chrome OS is currently setup to use the BFQ scheduler and I found that
>> I couldn't reproduce the problems without BFQ.  As discussed in the
>> second patch this is believed to be because BFQ sometimes returns
>> "true" from has_work() but then NULL from dispatch_request().
>>
>> I'll insert my usual caveat that I'm sending patches to code that I
>> know very little about.  If I'm making a total bozo patch here, please
>> help me figure out how I should fix the problems I found in a better
>> way.
>>
>> If you want to see a total ridiculous amount of chatter where I
>> stumbled around a whole bunch trying to figure out what was wrong and
>> how to fix it, feel free to read <https://crbug.com/1061950>.  I
>> promise it will make your eyes glaze over right away if this cover
>> letter didn't already do that.  Specifically comment 79 in that bug
>> includes a link to my ugly prototype of making BFQ's has_work() more
>> exact (I only managed it by actually defining _both_ an exact and
>> inexact function to avoid circular locking problems when it was called
>> directly from blk_mq_hctx_has_pending()).  Comment 79 also has more
>> thoughts about alternatives considered.
>>
>> I don't know if these fixes represent a regression of some sort or are
>> new.  As per above I could only reproduce with BFQ enabled which makes
>> it nearly impossible to go too far back with this.  I haven't listed
>> any "Fixes" tags here, but if someone felt it was appropriate to
>> backport this to some stable trees that seems like it'd be nice.
>> Presumably at least 5.4 stable would make sense.
>>
>> Thanks to Salman Qazi, Paolo Valente, and Guenter Roeck who spent a
>> bunch of time helping me trawl through some of this code and reviewing
>> early versions of this patch.
>>
>> Changes in v4:
>> - Only kick in blk_mq_do_dispatch_ctx() / blk_mq_do_dispatch_sched().
>>
>> Changes in v3:
>> - Note why blk_mq_dispatch_rq_list() change is needed.
>> - ("blk-mq: Add blk_mq_delay_run_hw_queues() API call") new for v3
>> - Always kick when putting the budget.
>> - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work().
>> - Totally rewrote commit message.
>> - ("Revert "scsi: core: run queue...") new for v3.
>>
>> Changes in v2:
>> - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...")
>>
>> Douglas Anderson (4):
>>   blk-mq: In blk_mq_dispatch_rq_list() "no budget" is a reason to kick
>>   blk-mq: Add blk_mq_delay_run_hw_queues() API call
>>   blk-mq: Rerun dispatching in the case of budget contention
>>   Revert "scsi: core: run queue if SCSI device queue isn't ready and
>>     queue is idle"
>>
>>  block/blk-mq-sched.c    | 18 ++++++++++++++++++
>>  block/blk-mq.c          | 30 +++++++++++++++++++++++++++---
>>  drivers/scsi/scsi_lib.c |  7 +------
>>  include/linux/blk-mq.h  |  1 +
>>  4 files changed, 47 insertions(+), 9 deletions(-)
> 
> Is there anything blocking this series from landing?  All has been
> quiet for a while.  All the patches have Ming's review and the SCSI
> patch has Martin's Ack.  This seems like a great time to get it into
> linux-next so it can get a whole bunch of testing before the next
> merge window.

Current series doesn't apply - can you resend it?

-- 
Jens Axboe

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