lkml.org 
[lkml]   [2012]   [May]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC][PATCH 4/6] arm, mm: Convert arm to generic tlb
On Thu, May 17, 2012 at 06:22:15PM +0100, Russell King wrote:
> On Thu, May 17, 2012 at 05:00:12PM +0100, Catalin Marinas wrote:
> > On Thu, May 17, 2012 at 12:28:06PM +0100, Peter Zijlstra wrote:
> > > On Thu, 2012-05-17 at 10:51 +0100, Russell King wrote:
> > > > On Thu, May 17, 2012 at 10:30:23AM +0100, Catalin Marinas wrote:
> > > > > Another minor thing is that on newer ARM processors (Cortex-A15) we
> > > > > need the TLB shootdown even on UP systems, so tlb_fast_mode should
> > > > > always return 0. Something like below (untested):
> > > >
> > > > No Catalin, we need this for virtually all ARMv7 CPUs whether they're UP
> > > > or SMP, not just for A15, because of the speculative prefetch which can
> > > > re-load TLB entries from the page tables at _any_ time.
> > >
> > > Hmm,. so this is mostly because of the confusion/coupling between
> > > tlb_remove_page() and tlb_remove_table() I guess. Since I don't see the
> > > freeing of the actual pages being a problem with speculative TLB
> > > reloads, just the page-tables.
> > >
> > > Should we introduce a tlb_remove_table() regardless of
> > > HAVE_RCU_TABLE_FREE which always queues the tables regardless of
> > > tlb_fast_mode()?
> >
> > BTW, looking at your tlb-unify branch, does tlb_remove_table() call
> > tlb_flush/tlb_flush_mmu before freeing the tables? I can only see
> > tlb_remove_page() doing this. On ARM, even UP, we need the TLB flushing
> > after clearing the pmd and before freeing the pte page table (and
> > ideally doing it less often than at every pte_free_tlb() call).
>
> Catalin,
>
> The way TLB shootdown stuff works is that _every_ single bit of memory
> which gets freed, whether its a page or a page table, gets added to a
> list of pages to be freed.
>
> So, the sequence is:
> - remove pte/pmd/pud/pgd pointers
> - add pages, whether they be pages pointed to by pte entries or page tables
> to be freed to a list
> - when list is sufficiently full, invalidate TLBs
> - free list of pages
>
> That means the pages will not be freed, whether it be a page mapped
> into userspace or a page table until such time that the TLB has been
> invalidated.
>
> For page tables, this is done via pXX_free_tlb(), which then calls out
> to the arch specific __pXX_free_tlb(), which ultimately then hands the
> page table over to tlb_remove_page() to add to the list of to-be-freed
> pages.

I know that already, not sure why you explained it again (but it's good
for future reference).

My point was that if we move to HAVE_RCU_FREE_TLB, the other
architectures doing this are calling tlb_remove_table() instead of
tlb_remove_page() in __p??_free_tlb(). And tlb_remove_table() does not
do any TLB maintenance when it can no longer queue pages (batch table
overflow).

--
Catalin


\
 
 \ /
  Last update: 2012-05-17 21:41    [W:1.360 / U:0.200 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site