linux-kernel - Re: [PATCH 2/2] x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount

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Message-ID: <20161126210633.GA5415@linux.intel.com>
Date:   Sat, 26 Nov 2016 13:06:34 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        Ingo Molnar <mingo@...e.hu>, Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Sai Prakhya <sai.praneeth.prakhya@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] x86/intel_rdt: Update task closid immediately on CPU
 in rmdir and unmount

On Sat, Nov 26, 2016 at 10:08:57AM +0100, Thomas Gleixner wrote:
> On Fri, 25 Nov 2016, Fenghua Yu wrote:
> > On Wed, Nov 23, 2016 at 03:23:50PM +0100, Thomas Gleixner wrote:
> > > +#ifdef CONFIG_SMP
> > > +			/*
> > > +			 * This is safe on x86 w/o barriers as the ordering
> > > +			 * of writing to task_cpu() and t->on_cpu is
> > > +			 * reverse to the reading here. The detection is
> > > +			 * inaccurate as tasks might move or schedule
> > > +			 * before the smp function call takes place. In
> > > +			 * such a case the function call is pointless, but
> > > +			 * there is no other side effect.
> > > +			 */
> > 
> > If process p1 is running on CPU1 before this point,
> > 
> > > +			if (mask && t->on_cpu)
> > > +				cpumask_set_cpu(task_cpu(t), mask);
> > 
> > If between CPU1 is set in mask and rdt_update_closid(tmpmask, NULL) is
> > called, p1 is switched to CPU2, and process p2 with its own closid
> > (e.g. 2) is switched to CPU1.
> > 
> > Then closid in PQR_ASSOC is set incorrectly as 0 instead of 2 on CPU1.
> > 0 may stay in PQR_ASSOC until next context switch which may take long time
> > in cases of real time or HPC.
> > 
> > Don't we need to care this situation? In this situation, the function call
> > is not "pointless" but it's wrong, right?
> 
> No.
> 
> CPU0		CPU1			CPU2
> 		T1 (closid 0)		T2 (closid 2)
> 
> (t1->on_cpu)
>   set(1, mask)
> 		preemption
> 		   T1 ->CPU2
> 		switch_to(T3)		preemption
> 		switch_to(idle)
> 					T2 -> CPU1
> 		switch_to(T2)		switch_to(T1)
> 		 intel_rdt_sched_in()	 intel_rdt_sched_in()
> 		  closid = T2->closid	 closid = T1->closid
> 		  closid =2 	  	 closid = CPU2->closid
> 				 				 
> rdt_update_closid(mask)
> 		
> 		rdt_update_cpu_closid()
> 		  intel_rdt_sched_in()
> 		   closid = T2->closid
> 		   closid = 2
> 
> IOW, whatever comes first, sched_switch() or function call will update the
> closid to the correct value.
> 
> If CPU2 was in the removed group then this looks the following way:
> 
> CPU0		CPU1			CPU2
> 		T1 (closid 0)		T2 (closid 2)
> 
> (t1->on_cpu)
>   set(1, mask)
> 		preemption
> 		   T1 ->CPU2
> 		switch_to(T3)		preemption
> 		switch_to(idle)
> 					T2 -> CPU1
> 		switch_to(T2)		switch_to(T1)
> 		 intel_rdt_sched_in()	 intel_rdt_sched_in()
> 		   closid = T2->closid	 closid = T1->closid (0)
> 		  closid =2 	  	 closid = CPU2->closid
> 		  	 		 closid = 5
> for_each_cpu(grp->mask)
> 	CPU2->closid = 0
> 		 
> rdt_update_closid(mask)
> 		
> 		rdt_update_cpu_closid()	rdt_update_cpu_closid()
> 		  intel_rdt_sched_in(	 intel_rdt_sched_in()
> 		   closid = T2->closid	  closid = T1->closid (0)
> 		   closid = 2		  closid = CPU2->closid
> 		   	    		  closid = 0
> 
> But on CPU2 the function call might be pointless as well in the following
> situation:
> 
> CPU0		CPU1			CPU2
> 		T1 (closid 0)		T2 (closid 2)
> 
> (t1->on_cpu)
>   set(1, mask)
> 		preemption
> 		   T1 ->CPU2
> 		switch_to(T3)		preemption
> 		switch_to(idle)
> 
> for_each_cpu(grp->mask)
> 	CPU2->closid = 0
> 					T2 -> CPU1
> 		switch_to(T2)		switch_to(T1)
> 		 intel_rdt_sched_in()	 intel_rdt_sched_in()
> 		   closid = T2->closid	 closid = T1->closid (0)
> 		  closid =2 	  	 closid = CPU2->closid
> 		  	 		 closid = 0
> 		 
> rdt_update_closid(mask)
> 		
> 		rdt_update_cpu_closid()	rdt_update_cpu_closid()
> 		  intel_rdt_sched_in(	 intel_rdt_sched_in()
> 		   closid = T2->closid	  closid = T1->closid (0)
> 		   closid = 2		  closid = CPU2->closid
> 		   	    		  closid = 0
> 
> The whole thing works by ordering:
> 
> 1) Update closids of each task in the group and if a task is running on a
>    cpu then mark the CPU on which the task is running for the function
>    call.
> 
> 2) Update closids of each CPU in the group
> 
> 3) Or the cpumasks of the tasks and the groups and invoke the function call
>    on all of them
> 
> If an affected task does a sched_switch after task->closid is updated and
> before the function call is invoked then the function call is pointless.
> 
> If a sched switch happens on a CPU after cpu->closid is updated and before
> the function call is invoked then the function call is pointless.
> 
> But there is no case where the function call can result in a wrong value.
> 
> Thanks,
> 
> 	tglx

Thank you for your clear explanation. You are absolutely correct. I know
I must miss something.

The reworked second patch and the first patch were tested successfully.
I assume you will check them in tip tree and I will not send v2.

Thanks.

-Fenghua