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

[Jerome Glisse <glisse@freedesktop.org>]

On Thu, Feb 18, 2010 at 04:30:32PM +0100, Jerome Glisse wrote:
> 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).

Ok, we are likely not hit by wc -> uc or uc -> wc change (which is dumb
but i haven't yet understand what is exactly happening for the user)
My guess is that on non PAT get_page_memtype always return -1
Thus i think we will need to export a function bool pat_is_enabled() so
ttm can pickup the proper path in the unlikely/broken case of
wc -> uc.

> 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