Messages in this thread | | | Subject | Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID | From | Nadav Amit <> | Date | Sat, 17 Jun 2017 23:26:48 -0700 |
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> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski <luto@kernel.org> 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Is this commit message outdated? NR_DYNAMIC_ASIDS is set to 6. More importantly, I do not see PCID 0 as reserved:
> +static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen, > + u16 *new_asid, bool *need_flush) > +{ >
[snip]
> + if (*new_asid >= NR_DYNAMIC_ASIDS) { > + *new_asid = 0; > + this_cpu_write(cpu_tlbstate.next_asid, 1); > + } > + *need_flush = true; > +}
Am I missing something?
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