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Re: [Qemu-block] [PATCH 0/4] throttle: Race condition fixes and test cas

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH 0/4] throttle: Race condition fixes and test cases
Date: Mon, 13 Aug 2018 13:42:33 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)


On Thu 02 Aug 2018 04:50:22 PM CEST, Alberto Garcia wrote:
> Hi all,
> here are the patches that I promised yesterday.
> I was originally thinking to propose this for the v3.0 release, but
> after debugging and fixing the problem I think that it's not
> essential (details below).
> The important patch is the second one. The first and the third are
> just test cases and the last is an alternative solution for the bug
> that Stefan fixed in 6fccbb475bc6effc313ee9481726a1748b6dae57.
> There are details in the patches themselves, but here's an explanation
> of the problem: consider a scenario with two drives A and B that are
> part of the same throttle group. Both of them have throttled requests
> and they're waiting for a timer that is set on drive A.
>     (timer here) -->  [A]  ---  req1, req2
>                       [B]  ---  req3
> If we drain drive [A] (e.g. by disabling its I/O limits) then its
> queue is restarted. req1 is processed immediately, and before
> finishing it calls schedule_next_request(). This follows the
> round-robin algorithm, selects req3 and puts a timer in [B].
> But we're still not done with draining [A], and now we have a
> BDRV_POLL_WHILE() loop at the end of bdrv_do_drained_begin() waiting
> for req2 to finish. That won't happen until the timer in [B] fires and
> req3 is done. If there are more drives in the group and more requests
> in the queue this can take a while. That's why disabling a drive's I/O
> limits can be noticeably slow: we disabled the I/O limits but they're
> still being enforced in practice.
> The QEMU I/O tests run in qtest mode (with QEMU_CLOCK_VIRTUAL). The
> clock must be advanced manually, which means that the scenario that I
> just described hangs QEMU because BDRV_POLL_WHILE() loops forever (you
> can reproduce this with patch 3). In a real world scenario this only
> results in the aforementioned slowdown (probably negligible in
> practice), which is not a critical thing, and that's why I think it's
> safe to keep the current code for QEMU 3.
> I think that's all. Questions and commend are welcome.
> Berto
> Alberto Garcia (4):
>   qemu-iotests: Test removing a throttle group member with a pending
>     timer
>   throttle-groups: Skip the round-robin if a member is being drained
>   qemu-iotests: Update 093 to improve the draining test
>   throttle-groups: Don't allow timers without throttled requests
>  block/throttle-groups.c    | 41 +++++++++++++++++++++++++---------
>  tests/qemu-iotests/093     | 55 
> ++++++++++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/093.out |  4 ++--
>  3 files changed, 88 insertions(+), 12 deletions(-)
> -- 
> 2.11.0

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