On Sat, 2019-04-06 at 16:13 -0400, Mikulas Patocka wrote: > > On Sat, 6 Apr 2019, James Bottomley wrote: > > > On Sat, 2019-04-06 at 15:36 -0400, Mikulas Patocka wrote: > > > Parisc uses a global spinlock to protect pagetable updates in the > TLB > > > fault handlers. When multiple cores are taking TLB faults > > > simultaneously, the cache line containing the spinlock becomes a > > > bottleneck. > > > > You can't do this. As the comment in cache.c says: the lock is to > > protect the merced bus, which runs between the CPUs on some > systems. > > That means it must be a single, global lock. > > So - how could we detect if the Merced bus is present? My best recollection is that it's only N class systems. > > Of course, on systems without a merced bus, we don't need the lock > at > > all, so runtime patching might be usable to fix that case. > > > > James > > The lock is still needed to synchronize TLB fault handlers with the > code that modifies the pagetables - but we could have per-process > lock for this purpose. It is? I don't think we need any per-arch sync for that. The purge should happen after all modifications are done so the next page fault inserts the new TLB entry ... so if there is a place where the purge lock matters to the page table updates, we're doing something wrong. James
