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Date: Tue, 8 Sep 2015 13:06:24 +0200
From: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20150908110624.GE4230@noname.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 4/5] disk_deadlines: add control of
 requests time expiration
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To: "Denis V. Lunev" <den@openvz.org>
Cc: Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, Raushaniya Maksudova <rmaksudova@virtuozzo.com>

Am 08.09.2015 um 10:00 hat Denis V. Lunev geschrieben:
> From: Raushaniya Maksudova <rmaksudova@virtuozzo.com>
> 
> If disk-deadlines option is enabled for a drive, one controls time
> completion of this drive's requests. The method is as follows (further
> assume that this option is enabled).
> 
> Every drive has its own red-black tree for keeping its requests.
> Expiration time of the request is a key, cookie (as id of request) is an
> appropriate node. Assume that every requests has 8 seconds to be completed.
> If request was not accomplished in time for some reasons (server crash or
> smth else), timer of this drive is fired and an appropriate callback
> requests to stop Virtial Machine (VM).
> 
> VM remains stopped until all requests from the disk which caused VM's
> stopping are completed. Furthermore, if there is another disks whose
> requests are waiting to be completed, do not start VM : wait completion
> of all "late" requests from all disks.
> 
> Signed-off-by: Raushaniya Maksudova <rmaksudova@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>

> +    disk_deadlines->expired_tree = true;
> +    need_vmstop = !atomic_fetch_inc(&num_requests_vmstopped);
> +    pthread_mutex_unlock(&disk_deadlines->mtx_tree);
> +
> +    if (need_vmstop) {
> +        qemu_system_vmstop_request_prepare();
> +        qemu_system_vmstop_request(RUN_STATE_PAUSED);
> +    }
> +}

What behaviour does this result in? If I understand correctly, this is
an indirect call of do_vm_stop(), which involves a bdrv_drain_all(). In
this case, qemu would completely block (including unresponsive monitor)
until the request can complete.

Is this what you are seeing with this patch, or why doesn't the
bdrv_drain_all() call cause such effects?

Kevin