Re: [Xen-devel] [PATCH] xen: make sure stop_machine_run() is always called in a tasklet

From: Igor Druzhinin <igor.druzhinin@citrix.com>
To: "Jürgen Groß" <jgross@suse.com>, xen-devel@lists.xenproject.org
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"George Dunlap" <George.Dunlap@eu.citrix.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"Jan Beulich" <jbeulich@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] xen: make sure stop_machine_run() is always called in a tasklet
Date: Fri, 14 Feb 2020 17:34:19 +0000	[thread overview]
Message-ID: <74d0b36a-4d80-39e1-aaf2-cf5209781e1e@citrix.com> (raw)
In-Reply-To: <f7397b04-80f2-1e2a-0899-427570f8f878@suse.com>

On 14/02/2020 16:39, Jürgen Groß wrote:
> On 14.02.20 15:06, Igor Druzhinin wrote:
>> On 11/02/2020 09:35, Juergen Gross wrote:
>>> With core scheduling active it is mandatory for stop_machine_run() to
>>> be called in a tasklet only, as otherwise a scheduling deadlock would
>>> occur: stop_machine_run() does a cpu rendezvous by activating a tasklet
>>> on all other cpus. In case stop_machine_run() was not called in an idle
>>> vcpu it would block scheduling the idle vcpu on its siblings with core
>>> scheduling being active, resulting in a hang.
>>
>> I suppose rcu_barrier() is fine due to process_pending_softirqs() being
>> called inside? I'm a little concerned by imposing is_vcpu_idle() restriction
>> in that case as rcu_barrier() could be technically called from a non-tasklet
>> context.
> 
> No, stop_machine_run() with core scheduling active can only work when
> called in an idle vcpu.
> 
> OTOH it would be fairly easy to add another softirq for a similar
> purpose and have a sync_machine_run() using that instead of tasklets.
> This could be used for ucode loading, too.
> 
> stop_machine_run() and sync_machine_run() could use a common main
> function. The patch should be rather simple.
> 
> Thoughts?

I have a patch on the list (which I was planning to send a v2 for) that
fixes another issue with rcu_barrier():
https://lists.xenproject.org/archives/html/xen-devel/2020-01/msg02273.html

As I understand it now that wouldn't work with core-scheduling. Do you think
it's possible to synchronously wait for tasklets to finish in non-tasklet
context (because that's what the purpose of rcu_barrier() is)?

Igor

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