On 9/22/20 7:56 PM, Ming Lei wrote:
On Tue, Sep 22, 2020 at 12:43:37PM +0800, JeffleXu wrote:
Thanks for replying. Comments embedded below.
On 9/13/20 10:00 PM, Ming Lei wrote:
On Fri, Sep 11, 2020 at 07:40:14PM +0800, JeffleXu wrote:
Thanks for replying ;)
On 9/11/20 7:01 PM, Ming Lei wrote:
On Fri, Sep 11, 2020 at 11:29:58AM +0800, Jeffle Xu wrote:
Splitted bios of one source bio can be enqueued into different CPU since
the submit_bio() routine can be preempted or fall asleep. However this
behaviour can't work well with iopolling.
Do you have user visible problem wrt. io polling? If yes, can you
provide more details?
No, there's no practical example yet. It's only a hint from the code base.
Currently block iopolling only polls the hardwar queue of the input bio.
If one bio is splitted to several bios, one (bio 1) of which is enqueued
into CPU A, while the others enqueued into CPU B, then the polling of bio 1
will cotinuously poll the hardware queue of CPU A, though the other
splitted bios may be in other hardware queues.
If it is guaranteed that the returned cookie is from bio 1, poll is
supposed to work as expected, since bio 1 is the chained head of these
bios, and the whole fs bio can be thought as done when bio1 .end_bio
is called.
Yes, it is, thanks for your explanation. But except for polling if the input
bio has completed, one of the
important work of polling logic is to reap the completion queue. Let's say
one bio is split into
two bios, bio 1 and bio 2, both of which are enqueued into the same hardware
queue.When polling bio1,
though we have no idea about bio2 at all, the polling logic itself is still
reaping the completion queue of
this hardware queue repeatedly, in which case the polling logic still
stimulates reaping bio2.
Then what if these two split bios enqueued into two different hardware
queue? Let's say bio1 is enqueued
into hardware queue A, while bio2 is enqueued into hardware queue B. When
polling bio1, though the polling
logic is repeatedly reaping the completion queue of hardware queue A, it
doesn't help reap bio2. bio2 is reaped
by IRQ as usual. This certainly works currently, but this behavior may
deviate the polling design? I'm not sure.
In other words, if we can ensure that all split bios are enqueued into the
same hardware queue, then the polling
logic *may* be faster.
__submit_bio_noacct_mq() returns cookie from the last bio in current->bio_list, and
this bio should be the bio passed to __submit_bio_noacct_mq() when bio splitting happens.
Suppose CPU migration happens during bio splitting, the last bio should be
submitted to LLD much late than other bios, so when blk_poll() finds
completion on the hw queue of the last bio, usually other bios should
be completed already most of times.
Also CPU migration itself causes much bigger latency, so it is reasonable to
not expect good IO performance when CPU migration is involved. And CPU migration
on IO task shouldn't have been done frequently. That said it should be
fine to miss the poll in this situation.
Yes you're right. After diving into the code of nvme driver, currently nvme
driver indeed allocate interrupt for polling queues,
No, nvme driver doesn't allocate interrupt for poll queues, please see
nvme_setup_irqs().
Sorry I was wrong here. Indeed interrupts are disabled for IO queues in
polling mode. Then this can be a problem.
If CPU migration happens, separate split bios can be enqueued into
different polling hardware queues (e.g. queue 1
and queue 2). The caller is continuously polling on one of the polling
hardware queue (e.g. queue 1) indicated by the
returned cookie. If there's no other thread polling on the other
hardware queue (e.g. queue 2), the split bio on queue 2
will not be reaped since the interrupt of queue 2 is disabled. Finally
the completion of this bio (on queue 2) relies on
timeout mechanism.
that is, reusing the interrupt used by admin queue.
Jens had ever said that the interrupt may be disabled for queues working in
polling mode someday (from my colleague). If
that is true, then this may become an issue. But at least now this indeed
works.
What is the issue?
Same issue described above.
Thanks,
Ming
