Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using PCID

[Peter Zijlstra]

On Thu, Jun 29, 2017 at 08:53:22AM -0700, Andy Lutomirski wrote:
> +static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> +                         u16 *new_asid, bool *need_flush)
> +{
> +     u16 asid;
> +
> +     if (!static_cpu_has(X86_FEATURE_PCID)) {
> +             *new_asid = 0;
> +             *need_flush = true;
> +             return;
> +     }
> +
> +     for (asid = 0; asid < TLB_NR_DYN_ASIDS; asid++) {
> +             if (this_cpu_read(cpu_tlbstate.ctxs[asid].ctx_id) !=
> +                 next->context.ctx_id)
> +                     continue;
> +
> +             *new_asid = asid;
> +             *need_flush = (this_cpu_read(cpu_tlbstate.ctxs[asid].tlb_gen) <
> +                            next_tlb_gen);
> +             return;
> +     }
> +
> +     /*
> +      * We don't currently own an ASID slot on this CPU.
> +      * Allocate a slot.
> +      */
> +     *new_asid = this_cpu_add_return(cpu_tlbstate.next_asid, 1) - 1;

So this basically RR the ASID slots. Have you tried slightly more
complex replacement policies like CLOCK ?

> +     if (*new_asid >= TLB_NR_DYN_ASIDS) {
> +             *new_asid = 0;
> +             this_cpu_write(cpu_tlbstate.next_asid, 1);
> +     }
> +     *need_flush = true;
> +}