Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

[Rik van Riel]

On 04/28/2015 06:54 PM, Andy Lutomirski wrote:
> On Tue, Apr 28, 2015 at 3:41 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>> On 04/28/2015 06:15 PM, Kirill A. Shutemov wrote:
>>> On Tue, Apr 28, 2015 at 01:42:10PM -0700, Andy Lutomirski wrote:
>>>> At some point, I'd like to implement PCID on x86 (if no one beats me
>>>> to it, and this is a low priority for me), which will allow us to skip
>>>> expensive TLB flushes while context switching.  I have no idea whether
>>>> ARM can do something similar.
>>>
>>> I talked with Dave about implementing PCID and he thinks that it will be
>>> net loss. TLB entries will live longer and it means we would need to trigger
>>> more IPIs to flash them out when we have to. Cost of IPIs will be higher
>>> than benifit from hot TLB after context switch.
>>
>> I suspect that may depend on how you do the shootdown.
>>
>> If, when receiving a TLB shootdown for a non-current PCID, we just flush
>> all the entries for that PCID and remove the CPU from the mm's
>> cpu_vm_mask_var, we will never receive more than one shootdown IPI for
>> a non-current mm, but we will still get the benefits of TLB longevity
>> when dealing with eg. pipe workloads where tasks take turns running on
>> the same CPU.
> 
> I had a totally different implementation idea in mind.  It goes
> something like this:
> 
> For each CPU, we allocate a fixed number of PCIDs, e.g. 0-7.  We have
> a per-cpu array of the mm [1] that owns each PCID.  On context switch,
> we look up the new mm in the array and, if there's a PCID mapped, we
> switch cr3 and select that PCID.  If there is no PCID mapped, we
> choose one (LRU?  clock replacement?), switch cr3 and select and
> invalidate that PCID.
> 
> When it's time to invalidate a TLB entry on an mm that's active
> remotely, we really don't want to send an IPI to a CPU that doesn't
> actually have that mm active.  Instead we bump some kind of generation
> counter in the mm_struct that will cause the next switch to that mm
> not to match the PCID list.  To keep this working, I think we also
> need to update the per-cpu PCID list with our generation counter
> either when we context switch out or when we process a TLB shootdown
> IPI.

If we do that, we can also get rid of TLB shootdowns for
idle CPUs in lazy TLB mode.

Very nice, if the details work out.

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