Il giorno 25 gen 2017, alle ore 17:13, Jens Axboe [off-list ref] ha =

scritto:

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On 01/25/2017 01:46 AM, Paolo Valente wrote:

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Il giorno 23 gen 2017, alle ore 18:42, Jens Axboe [off-list ref] ha =

scritto:

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On 01/23/2017 10:04 AM, Paolo Valente wrote:

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Il giorno 18 gen 2017, alle ore 17:21, Jens Axboe [off-list ref] =

ha scritto:

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On 01/18/2017 08:14 AM, Paolo Valente wrote:

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according to the function blk_mq_sched_put_request, the
mq.completed_request hook seems to always be invoked (if set) for =

a

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request for which the mq.put_rq_priv is invoked (if set).

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Correct, any request that came out of blk_mq_sched_get_request()
will always have completed called on it, regardless of whether it
had IO started on it or not.
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It seems that some request, after being dispatched, happens to have =

no

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mq.put_rq_priv invoked on it now or then.  Is it expected?  If it =

is,

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could you point me to the path through which the end of the life of
such a request is handled?

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I'm guessing that's a flush request. I added RQF_QUEUED to check for
that, if RQF_QUEUED is set, you know it has come from your =

get_request

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handler.
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Exactly, the completion-without-put_rq_priv pattern seems to occur
only for requests coming from the flusher, precisely because they =

have

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the flag RQF_ELVPRIV unset.  Just to understand: why is this flag
unset for these requests, if they do have private elevator (bfq)
data attached?  What am I misunderstanding?
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Just to be certain: this should be the only case where the
completed_request hook is invoked while the put_rq_priv is not, =

right?

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They must NOT have scheduler data attached. In your get_request
function, you must bypass if blk_mq_sched_bypass_insert() returns =

true.

Yes, sorry.  I'm already using blk_mq_sched_bypass_insert() to bypass
insertion in the insert hook, as done in mq-deadline, and I have no
get_request defined (see below).

The source of my confusion was that I assumed that flush requests had
not to leave any trace in the scheduler, since the scheduler does not
decide anything for them.  Accordingly, I thought they did not trigger
any put or completion hook.  In contrast, these requests get the
flag QUEUED set, in case the get_request hook is set, and then trigger
both a put_request and a completed_request.  In this respect, in bfq-mq
I'm not using any of these three hooks (they are all NULL).  I hope
I'm not doing something unexpected or incoherent.

UPDATE: bfq-mq now survives for minutes.  I'm debugging two occasional
failures, which (un)fortunately become more and more occasional as I
go on with debugging and instrumenting the code.

Thanks,
Paolo

See how mq-deadline does that. This is important, or you will get =

hangs

with flushes as well, since the IO scheduler private data and the =

flush

data is unionized in the request.
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Jens Axboe