linux-kernel - Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu

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Date:	Tue, 15 Sep 2015 23:11:45 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>, KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm

Am 15.09.2015 um 18:42 schrieb Paolo Bonzini:
> 
> 
> On 15/09/2015 15:36, Christian Borntraeger wrote:
>> I am wondering why the old code behaved in such fatal ways. Is there
>> some interaction between waiting for a reschedule in the
>> synchronize_sched writer and some fork code actually waiting for the
>> read side to get the lock together with some rescheduling going on
>> waiting for a lock that fork holds? lockdep does not give me an hints
>> so I have no clue :-(
> 
> It may just be consuming too much CPU usage.  kernel/rcu/tree.c warns
> about it:
> 
>  * if you are using synchronize_sched_expedited() in a loop, please
>  * restructure your code to batch your updates, and then use a single
>  * synchronize_sched() instead.
> 
> and you may remember that in KVM we switched from RCU to SRCU exactly to
> avoid userspace-controlled synchronize_rcu_expedited().
> 
> In fact, I would say that any userspace-controlled call to *_expedited()
> is a bug waiting to happen and a bad idea---because userspace can, with
> little effort, end up calling it in a loop.

Right. This also implies that we should fix this for 4.2 - I guess.

Christian

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