linux-kernel - Re: [PATCH v3 11/11] x86/mm: Try to preserve old TLB entries using PCID

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Date:   Wed, 21 Jun 2017 15:40:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 11/11] x86/mm: Try to preserve old TLB entries using
 PCID

On Wed, 21 Jun 2017, Thomas Gleixner wrote:
> > +	for (asid = 0; asid < NR_DYNAMIC_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;
> > +	}
> 
> Hmm. So this loop needs to be taken unconditionally even if the task stays
> on the same CPU. And of course the number of dynamic IDs has to be short in
> order to makes this loop suck performance wise.

 ...  not suck ...