linux-kernel - Uncool feature for TTM introduced by x86, pat: Use page flags to track memtypes of RAM pages

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Message-ID: <20100218153032.GA4506@localhost.localdomain>
Date:	Thu, 18 Feb 2010 16:30:32 +0100
From:	Jerome Glisse <glisse@...edesktop.org>
To:	hpa@...or.com, suresh.b.siddha@...el.com,
	venkatesh.pallipadi@...el.com
Cc:	thellstrom@...are.com, airlied@...ux.ie, currojerez@...eup.net,
	linux-kernel@...r.kernel.org
Subject: Uncool feature for TTM introduced by x86, pat: Use page flags to
 track memtypes of RAM pages

Hi,

commit id: f58417409603d62f2eb23db4d2cf6853d84a1698

TTM is doing uncommon use of set_memory_wc|uc|wb for instance it's not
uncommon for TTM to change memory type from wc to uc or from uc to wc.
Since x86, pat: Use page flags to track memtypes of RAM pages (commit
id above) this isn't allowed anymore, before going from uc to wc or
wc to uc we first have to free the memtype by going through wb this
means an extra step which likely lead to some overhead (i guess that
uc -> wc or wc -> uc won't trigger massive tlb/cpu flush while
uc -> wb -> wc or wc -> wb -> uc will). reserve_ram_pages_type is the
function which will check that memory is wb thus enforcing us to go
through wb step.

Can we modify the interface to support again changing from uc to wc
or wc to uc ? (i can try to do a patch for that).

If no, we have a sever regression on non PAT arch :
http://bugzilla.kernel.org/show_bug.cgi?id=15328
(AFAIK bugzilla.kernel.org is down for me at the moment) because we
are doing the extra set wb step (i haven't dived deep into the code
to check what happen on non PAT). My guess is that on non PAT the
extra set wb can be avoided right ? Also what is your educated guess
on why on non PAT this extra set wb is slowing down thing badly ?
Last can we make this extra step only on PAT enabled arch ?

In PAT case right now we are calling get get_page_memtype to know
if we need to set page to wb first my understanding is that we should
protect this call by memtype_lock spinlock (even if i don't think we
can have collision here as we are protected by TTM locking), maybe
we can directly use TTM information to call or not set wb and avoid
using get_page_memtype.

Sorry for being such annoying user of PAT, i guess GPU is the only
place having such strange and intensive cache change pattern.

Cheers,
Jerome
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