linux-kernel - Re: [mm PATCH v6 6/7] mm: Add reserved flag setting to set_page

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Message-ID: <20181205204247.GY1286@dhcp22.suse.cz>
Date:   Wed, 5 Dec 2018 21:42:47 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, davem@...emloft.net,
        pavel.tatashin@...rosoft.com, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com,
        dave.jiang@...el.com, rppt@...ux.vnet.ibm.com, willy@...radead.org,
        vbabka@...e.cz, khalid.aziz@...cle.com, ldufour@...ux.vnet.ibm.com,
        mgorman@...hsingularity.net, yi.z.zhang@...ux.intel.com
Subject: Re: [mm PATCH v6 6/7] mm: Add reserved flag setting to set_page_links

On Wed 05-12-18 09:55:17, Alexander Duyck wrote:
> On Wed, 2018-12-05 at 18:22 +0100, Michal Hocko wrote:
> > On Fri 30-11-18 13:53:18, Alexander Duyck wrote:
> > > Modify the set_page_links function to include the setting of the reserved
> > > flag via a simple AND and OR operation. The motivation for this is the fact
> > > that the existing __set_bit call still seems to have effects on performance
> > > as replacing the call with the AND and OR can reduce initialization time.
> > > 
> > > Looking over the assembly code before and after the change the main
> > > difference between the two is that the reserved bit is stored in a value
> > > that is generated outside of the main initialization loop and is then
> > > written with the other flags field values in one write to the page->flags
> > > value. Previously the generated value was written and then then a btsq
> > > instruction was issued.
> > > 
> > > On my x86_64 test system with 3TB of persistent memory per node I saw the
> > > persistent memory initialization time on average drop from 23.49s to
> > > 19.12s per node.
> > 
> > I have tried to explain why the whole reserved bit doesn't make much
> > sense in this code several times already. You keep ignoring that and
> > that is highly annoying. Especially when you add a tricky code to
> > optimize something that is not really needed.
> > 
> > Based on that I am not going to waste my time on other patches in this
> > series to review and give feedback which might be ignored again.
> 
> I got your explanation. However Andrew had already applied the patches
> and I had some outstanding issues in them that needed to be addressed.
> So I thought it best to send out this set of patches with those fixes
> before the code in mm became too stale. I am still working on what to
> do about the Reserved bit, and plan to submit it as a follow-up set.

>From my experience Andrew can drop patches between different versions of
the patchset. Things can change a lot while they are in mmotm and under
the discussion.
-- 
Michal Hocko
SUSE Labs