On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > >> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> PCID is a "process context ID" -- it's what other architectures call >> an address space ID. Every non-global TLB entry is tagged with a >> PCID, only TLB entries that match the currently selected PCID are >> used, and we can switch PGDs without flushing the TLB. x86's >> PCID is 12 bits. >> >> This is an unorthodox approach to using PCID. x86's PCID is far too >> short to uniquely identify a process, and we can't even really >> uniquely identify a running process because there are monster >> systems with over 4096 CPUs. To make matters worse, past attempts >> to use all 12 PCID bits have resulted in slowdowns instead of >> speedups. >> >> This patch uses PCID differently. We use a PCID to identify a >> recently-used mm on a per-cpu basis. An mm has no fixed PCID >> binding at all; instead, we give it a fresh PCID each time it's >> loaded except in cases where we want to preserve the TLB, in which >> case we reuse a recent value. >> >> In particular, we use PCIDs 1-3 for recently-used mms and we reserve >> PCID 0 for swapper_pg_dir and for PCID-unaware CR3 users (e.g. EFI). >> Nothing ever switches to PCID 0 without flushing PCID 0 non-global >> pages, so PCID 0 conflicts won't cause problems. > > Is this commit message outdated? Yes, it's old. Will fix. -- 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>
