Date: Wed, 21 Jun 2017 15:40:58 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
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>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 11/11] x86/mm: Try to preserve old TLB entries using
 PCID
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References: <cover.1498022414.git.luto@kernel.org> <a8cdfbbb17785aed10980d24692745f68615a584.1498022414.git.luto@kernel.org> <alpine.DEB.2.20.1706211159430.2328@nanos>
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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 ...