Re: [PATCH] sched,x86: optimize switch_mm for multi-threaded workloads

[Paul Turner]

We attached the following explanatory comment to our version of the patch:

/*
* In the common case (two user threads sharing mm
* switching) the bit will be set; avoid doing a write
* (via atomic test & set) unless we have to.  This is
* safe, because no other CPU ever writes to our bit
* in the mask, and interrupts are off (so we can't
* take a TLB IPI here.)  If we don't do this, then
* switching threads will pingpong the cpumask
* cacheline.
*/

On Wed, Jul 31, 2013 at 4:12 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> On 07/31/2013 06:46 PM, Linus Torvalds wrote:
>>
>>
>> On Jul 31, 2013 3:39 PM, "Rik van Riel" <riel@xxxxxxxxxx
>>
>> <mailto:riel@xxxxxxxxxx>> wrote:
>>  >
>>  > On 07/31/2013 06:21 PM, Linus Torvalds wrote:
>>  >>
>>  >> Ummm.. The race is to the testing of the bit, not setting. The testing
>>  >> of the bit is not valid before we have set the tlb state, AFAIK.
>>  >
>>  >
>>  > I believe the bit is cleared and set by the current CPU.
>>
>> Yes, but we need to be careful with interrupts.
>>
>>
>>  > Interrupts are blocked inside switch_mm, so I think we
>>  > are safe.
>>
>> Are they? I thought we removed all that.
>
>
> context_switch() shows that the runqueue lock (which is an irq
> lock) is released, and irqs re-enabled, by the next task, after
> switch_to(), in finish_lock_switch(), called from finish_task_switch()
>
>> Note that switch_mm gets called for activate_mm too, or something.
>
>
> Good catch, though it looks like activate_mm is only called from
> exec_mmap, with the new mm as its argument.  While the new mm can
> have pages in memory yet, it has never been run so there should
> be nothing in the TLB yet for the new mm.
>
> This is subtler than I thought, but it does appear to be safe.
>
>
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