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

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Date:   Wed, 5 Jul 2017 14:25:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using
 PCID

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;
> +}