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Date: Wed, 5 Jul 2017 14:25:06 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Nadav Amit <nadav.amit@gmail.com>, Rik van Riel <riel@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [PATCH v4 10/10] x86/mm: Try to preserve old TLB entries using
 PCID
Message-ID: <20170705122506.GG4941@worktop>
References: <cover.1498751203.git.luto@kernel.org>
 <cf600d28712daa8e2222c08a10f6c914edab54f2.1498751203.git.luto@kernel.org>
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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;
> +}