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. -- All rights reversed -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>