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

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Date:	Wed, 16 Sep 2015 14:56:12 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm

Am 16.09.2015 um 14:22 schrieb Oleg Nesterov:
>>  The issue is that rcu_sync doesn't eliminate synchronize_sched,
> 
> Yes, but it eliminates _expedited(). This is good, but otoh this means
> that (say) individual __cgroup_procs_write() can take much more time.
> However, it won't block the readers and/or disturb the whole system.
> And percpu_up_write() doesn't do synchronize_sched() at all.
> 
>> it only
>> makes it more rare.
> 
> Yes, so we can hope that multiple __cgroup_procs_write()'s can "share"
> a single synchronize_sched().

And in fact it does. Paolo suggested to trace how often we call 
synchronize_sched so I applied some advanced printk debugging technology ;-)
Until login I have 41 and after starting the 70 guests this went up to 48.
Nice work.

> 
>> So it's possible that it isn't eliminating the root
>> cause of the problem.
> 
> We will see... Just in case, currently the usage of percpu_down_write()
> is suboptimal. We do not need to do ->sync() under cgroup_mutex. But
> this needs some WIP changes in rcu_sync. Plus we can do more improvements,
> but this is off-topic right now.
> 
> Oleg.
> 

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