linux-kernel - Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

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Message-ID: <39dfc425-deff-2469-7bcb-4a0e177b31d1@linux.ibm.com>
Date:   Mon, 26 Sep 2022 14:32:24 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     peterz@...radead.org
Cc:     bigeasy@...utronix.de, dietmar.eggemann@....com,
        ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, mgorman@...e.de, mingo@...nel.org,
        oleg@...hat.com, rjw@...ysocki.net, rostedt@...dmis.org,
        tj@...nel.org, vincent.guittot@...aro.org, will@...nel.org,
        Marc Hartmayer <mhartmay@...ux.ibm.com>,
        Amit Shah <amit@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic



Am 26.09.22 um 12:55 schrieb Christian Borntraeger:
> 
> 
> Am 26.09.22 um 10:06 schrieb Christian Borntraeger:
>>
>>
>> Am 23.09.22 um 09:53 schrieb Christian Borntraeger:
>>> Am 23.09.22 um 09:21 schrieb Christian Borntraeger:
>>>> Peter,
>>>>
>>>> as a heads-up. This commit (bisected and verified) triggers a
>>>> regression in our KVM on s390x CI. The symptom is that a specific
>>>> testcase (start a guest with next kernel and a poky ramdisk,
>>>> then ssh via vsock into the guest and run the reboot command) now
>>>> takes much longer (300 instead of 20 seconds). From a first look
>>>> it seems that the sshd takes very long to end during shutdown
>>>> but I have not looked into that yet.
>>>> Any quick idea?
>>>>
>>>> Christian
>>>
>>> the sshd seems to hang in virtio-serial (not vsock).
>>
>> FWIW, sshd does not seem to hang, instead it seems to busy loop in
>> wait_port_writable calling into the scheduler over and over again.
> 
> -#define TASK_FREEZABLE                 0x00002000
> +#define TASK_FREEZABLE                 0x00000000
> 
> "Fixes" the issue. Just have to find out which of users is responsible.

So it seems that my initial test was not good enough.

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 9fa3c76a267f..e93df4f735fe 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -790,7 +790,7 @@ static int wait_port_writable(struct port *port, bool nonblock)
                 if (nonblock)
                         return -EAGAIN;
  
-               ret = wait_event_freezable(port->waitqueue,
+               ret = wait_event_interruptible(port->waitqueue,
                                            !will_write_block(port));
                 if (ret < 0)
                         return ret;

Does fix the problem.
My initial test was the following

--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -790,10 +790,8 @@ static int wait_port_writable(struct port *port, bool nonblock)
                 if (nonblock)
                         return -EAGAIN;
  
-               ret = wait_event_freezable(port->waitqueue,
+               wait_event(port->waitqueue,
                                            !will_write_block(port));
-               if (ret < 0)
-                       return ret;
         }
         /* Port got hot-unplugged. */
         if (!port->guest_connected)

and obviously it did not provide an exit path.