linux-kernel - Re: [PATCH v2 5/5] perf/x86/amd/power: Add AMD accumulated power reporting mechanism

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Message-ID: <20160121155120.GI6357@twins.programming.kicks-ass.net>
Date:	Thu, 21 Jan 2016 16:51:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Huang Rui <ray.huang@....com>
Cc:	Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <rric@...nel.org>,
	Jacob Shin <jacob.w.shin@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	Fr�d�ric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, spg_linux_kernel@....com,
	x86@...nel.org, Guenter Roeck <linux@...ck-us.net>,
	Andreas Herrmann <herrmann.der.user@...glemail.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	Borislav Petkov <bp@...en8.de>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH v2 5/5] perf/x86/amd/power: Add AMD accumulated power
 reporting mechanism

On Thu, Jan 21, 2016 at 11:24:14PM +0800, Huang Rui wrote:
> On Thu, Jan 21, 2016 at 04:10:40PM +0100, Peter Zijlstra wrote:
> > On Thu, Jan 21, 2016 at 10:42:35PM +0800, Huang Rui wrote:
> > > > > @@ -164,14 +171,14 @@ static int pmu_event_add(struct perf_event *event, int mode)
> > > > >  	struct hw_perf_event *hwc = &event->hw;
> > > > >  	unsigned long flags;
> > > > >  
> > > > > -	spin_lock_irqsave(&pmu->lock, flags);
> > > > > +	raw_spin_lock_irqsave(&pmu->lock, flags);
> > > > >  
> > > > >  	hwc->state = PERF_HES_UPTODATE | PERF_HES_STOPPED;
> > > > >  
> > > > >  	if (mode & PERF_EF_START)
> > > > >  		__pmu_event_start(pmu, event);
> > > > >  
> > > > > -	spin_unlock_irqrestore(&pmu->lock, flags);
> > > > > +	raw_spin_unlock_irqrestore(&pmu->lock, flags);
> > > > >  
> > > > >  	return 0;
> > > > >  }
> > > > 
> > > > So for these 4 {start,stop,add,del} you can drop the irqsave/irqrestore
> > > > thing as its guaranteed that IRQs will be disabled.
> > > > 
> > > 
> > > OK, I will remove the lock.
> > 
> > No, the lock seems needed, as the list is global. Just the
> > irqsave/irqrestore part is superfluous.
> > 
> 
> But actually, the lock is only used at {start,stop,add,del}. If we
> drop irqsave/irqrestore on these 4 things, there won't be any use
> cases.

But, but, the list is global !? something needs to serialize the access
to it, right?