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From: Ira Weiny <ira.weiny@intel.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-doc@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	Dan Williams <dan.j.williams@intel.com>,
	Greg KH <gregkh@linuxfoundation.org>
Subject: Re: [PATCH V3 10/10] x86/pks: Add PKS test code
Date: Thu, 17 Dec 2020 20:05:09 -0800	[thread overview]
Message-ID: <20201218040509.GD1563847@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <570ead2a-ff41-e730-d61d-0f59c67b1903@intel.com>

On Thu, Dec 17, 2020 at 12:55:39PM -0800, Dave Hansen wrote:
> On 11/6/20 3:29 PM, ira.weiny@intel.com wrote:
> > +		/* Arm for context switch test */
> > +		write(fd, "1", 1);
> > +
> > +		/* Context switch out... */
> > +		sleep(4);
> > +
> > +		/* Check msr restored */
> > +		write(fd, "2", 1);
> 
> These are always tricky.  What you ideally want here is:
> 
> 1. Switch away from this task to a non-PKS task, or
> 2. Switch from this task to a PKS-using task, but one which has a
>    different PKS value

Or both...

> 
> then, switch back to this task and make sure PKS maintained its value.
> 
> *But*, there's no absolute guarantee that another task will run.  It
> would not be totally unreasonable to have the kernel just sit in a loop
> without context switching here if no other tasks can run.
> 
> The only way you *know* there is a context switch is by having two tasks
> bound to the same logical CPU and make sure they run one after another.

Ah...  We do that.

...
+       CPU_ZERO(&cpuset);
+       CPU_SET(0, &cpuset);
+       /* Two processes run on CPU 0 so that they go through context switch.  */
+       sched_setaffinity(getpid(), sizeof(cpu_set_t), &cpuset);
...

I think this should be ensuring that both the parent and the child are
running on CPU 0.  At least according to the man page they should be.

<man>
	A child created via fork(2) inherits its parent's CPU affinity mask.
</man>

Perhaps a better method would be to synchronize the 2 threads more to ensure
that we are really running at the 'same time' and forcing the context switch.

>  This just gets itself into a state where it *CAN* context switch and
> prays that one will happen.

Not sure what you mean by 'This'?  Do you mean that running on the same CPU
will sometimes not force a context switch?  Or do you mean that the sleeps
could be badly timed and the 2 threads could run 1 after the other on the same
CPU?  The latter is AFAICT the most likely case.

> 
> You can also run a bunch of these in parallel bound to a single CPU.
> That would also give you higher levels of assurance that *some* context
> switch happens at sleep().

I think more cycles is a good idea for sure.  But I'm more comfortable with
forcing the test to be more synchronized so that it is actually running in the
order we think/want it to be.

> 
> One critical thing with these tests is to sabotage the kernel and then
> run them and make *sure* they fail.  Basically, if you screw up, do they
> actually work to catch it?

I'll try and come up with a more stressful test.

Ira

  reply	other threads:[~2020-12-18  4:06 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-06 23:28 [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 ira.weiny
2020-11-06 23:28 ` [PATCH V3 01/10] x86/pkeys: Create pkeys_common.h ira.weiny
2020-11-06 23:29 ` [PATCH V3 02/10] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support ira.weiny
2020-11-06 23:29 ` [PATCH V3 03/10] x86/pks: Add PKS defines and Kconfig options ira.weiny
2020-11-06 23:29 ` [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch ira.weiny
2020-12-17 14:50   ` Thomas Gleixner
2020-12-17 22:43     ` Thomas Gleixner
2020-12-18 13:57       ` Thomas Gleixner
2020-12-18 19:20         ` Dan Williams
2020-12-18 21:06           ` Thomas Gleixner
2020-12-18 21:58             ` Dan Williams
2020-12-18 22:44               ` Thomas Gleixner
2020-12-18 19:42         ` Ira Weiny
2020-12-18 20:10           ` Dave Hansen
2020-12-18 21:30           ` Thomas Gleixner
2020-12-18  4:05     ` Ira Weiny
2020-12-17 20:41   ` [NEEDS-REVIEW] " Dave Hansen
2020-12-18  4:10     ` Ira Weiny
2020-12-18 15:33       ` Dave Hansen
2020-11-06 23:29 ` [PATCH V3 05/10] x86/entry: Pass irqentry_state_t by reference ira.weiny
2020-11-15 18:58   ` Thomas Gleixner
2020-11-16 18:49     ` Ira Weiny
2020-11-16 20:36       ` Thomas Gleixner
2020-11-24  6:09   ` [PATCH V3.1] entry: " ira.weiny
2020-12-11 22:14     ` Andy Lutomirski
2020-12-16  1:32       ` Ira Weiny
2020-12-16  2:09         ` Andy Lutomirski
2020-12-17  0:38           ` Ira Weiny
2020-12-17 13:07       ` Thomas Gleixner
2020-12-17 13:19         ` Peter Zijlstra
2020-12-17 15:35           ` Andy Lutomirski
2020-12-17 16:58     ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions ira.weiny
2020-12-17 15:28   ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 07/10] x86/fault: Report the PKRS state on fault ira.weiny
2020-11-06 23:29 ` [PATCH V3 08/10] x86/pks: Add PKS kernel API ira.weiny
2020-12-23 20:39   ` Randy Dunlap
2020-11-06 23:29 ` [PATCH V3 09/10] x86/pks: Enable Protection Keys Supervisor (PKS) ira.weiny
2020-11-06 23:29 ` [PATCH V3 10/10] x86/pks: Add PKS test code ira.weiny
2020-12-17 20:55   ` Dave Hansen
2020-12-18  4:05     ` Ira Weiny [this message]
2020-12-18 16:59       ` Dan Williams
2020-12-07 22:14 ` [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 Ira Weiny
2020-12-08 15:55   ` Thomas Gleixner
2020-12-08 17:22     ` Ira Weiny

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