Date: Wed, 19 Oct 2005 11:06:32 +0200
From: Andrea Arcangeli <andrea@suse.de>
To: Andrew Morton <akpm@osdl.org>
Cc: linux-kernel@vger.kernel.org, olof@austin.ibm.com
Subject: Re: [PATCH] .text page fault SMP scalability optimization
Message-ID: <20051019090632.GC30541@x30.random>
References: <20051019075255.GB30541@x30.random> <20051019011420.032ccd6d.akpm@osdl.org>
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On Wed, Oct 19, 2005 at 01:14:20AM -0700, Andrew Morton wrote:
> How strange.  Do you mena that all CPUs were entering the pagefault handler
> on behalf of the same pte all the time?  That they're staying in lockstep?

Yes.

> If so, there must be a bunch of page_table_lock contention too?

Not really as much. Note also that with latest mainline the ppc64 kernel
was going well even without this patch, it was some older codebase
falling apart, primarly because it was still doing pte_establish there
see:

	young_entry = pte_mkyoung(entry);
	if (!pte_same(young_entry, entry)) {
		ptep_establish(vma, address, pte, young_entry);
		update_mmu_cache(vma, address, young_entry);
		lazy_mmu_prot_update(young_entry);
	}

So those flush actions were a bottleneck when executed by all cpus at
the same time. But some archs still implement it like with the old
codebase, and the patch is quite an obvious optimization that can
clearly avoid useless tlb flushes (and that fixed the problem completely
with the older codebase still dong ptep_establish even on ppc64).
-
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