From: Leonardo Bras <leonardo@linux.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Song Liu" <songliubraving@fb.com>,
"Michal Hocko" <mhocko@suse.com>,
"Dmitry V. Levin" <ldv@altlinux.org>,
"Keith Busch" <keith.busch@intel.com>,
linux-mm@kvack.org, "Paul Mackerras" <paulus@samba.org>,
"Christoph Lameter" <cl@linux.com>,
"Ira Weiny" <ira.weiny@intel.com>,
"Dan Williams" <dan.j.williams@intel.com>,
"Elena Reshetova" <elena.reshetova@intel.com>,
linux-arch@vger.kernel.org,
"Santosh Sivaraj" <santosh@fossix.org>,
"Davidlohr Bueso" <dave@stgolabs.net>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
"Mike Rapoport" <rppt@linux.ibm.com>,
"Jason Gunthorpe" <jgg@ziepe.ca>,
"Vlastimil Babka" <vbabka@suse.cz>,
"Mahesh Salgaonkar" <mahesh@linux.vnet.ibm.com>,
"Andrey Ryabinin" <aryabinin@virtuozzo.com>,
"Alexey Dobriyan" <adobriyan@gmail.com>,
"Ingo Molnar" <mingo@kernel.org>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Ralph Campbell" <rcampbell@nvidia.com>,
"Arnd Bergmann" <arnd@arndb.de>, "Jann Horn" <jannh@google.com>,
"John Hubbard" <jhubbard@nvidia.com>,
"Jesper Dangaard Brouer" <brouer@redhat.com>,
"Nicholas Piggin" <npiggin@gmail.com>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
kvm-ppc@vger.kernel.org, "Thomas Gleixner" <tglx@linutronix.de>,
"Reza Arbab" <arbab@linux.ibm.com>,
"Allison Randal" <allison@lohutok.net>,
"Christian Brauner" <christian.brauner@ubuntu.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org,
"Logan Gunthorpe" <logang@deltatee.com>,
"Souptick Joarder" <jrdr.linux@gmail.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
linuxppc-dev@lists.ozlabs.org, "Roman Gushchin" <guro@fb.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
"Al Viro" <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks
Date: Thu, 03 Oct 2019 18:24:07 -0300 [thread overview]
Message-ID: <c46ba8cec981ad28383bb7b23161fb83ccda4a60.camel@linux.ibm.com> (raw)
In-Reply-To: <20191003115141.GJ4581@hirez.programming.kicks-ass.net>
[-- Attachment #1: Type: text/plain, Size: 3835 bytes --]
Hello Peter, thanks for the feedback!
On Thu, 2019-10-03 at 13:51 +0200, Peter Zijlstra wrote:
> On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote:
> > > diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> > > index 818691846c90..3043ea9812d5 100644
> > > --- a/include/asm-generic/pgtable.h
> > > +++ b/include/asm-generic/pgtable.h
> > > @@ -1171,6 +1171,64 @@ static inline bool arch_has_pfn_modify_check(void)
> > > #endif
> > > #endif
> > >
> > > +#ifndef __HAVE_ARCH_LOCKLESS_PGTBL_WALK_CONTROL
> > > +static inline unsigned long begin_lockless_pgtbl_walk(struct mm_struct *mm)
> > > +{
> > > + unsigned long irq_mask;
> > > +
> > > + if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
> > > + atomic_inc(&mm->lockless_pgtbl_walkers);
> >
> > This will not work for file backed THP. Also, this is a fairly serious
> > contention point all on its own.
>
> Kiryl says we have tmpfs-thp, this would be broken vs that, as would
> your (PowerPC) use of mm_cpumask() for that IPI.
Could you please explain it?
I mean, why this breaks tmpfs-thp?
Also, why mm_cpumask() is also broken?
>
> > > + /*
> > > + * Interrupts must be disabled during the lockless page table walk.
> > > + * That's because the deleting or splitting involves flushing TLBs,
> > > + * which in turn issues interrupts, that will block when disabled.
> > > + */
> > > + local_irq_save(irq_mask);
> > > +
> > > + /*
> > > + * This memory barrier pairs with any code that is either trying to
> > > + * delete page tables, or split huge pages. Without this barrier,
> > > + * the page tables could be read speculatively outside of interrupt
> > > + * disabling.
> > > + */
> > > + smp_mb();
> >
> > I don't think this is something smp_mb() can guarantee. smp_mb() is
> > defined to order memory accesses, in this case the store of the old
> > flags vs whatever comes after this.
> >
> > It cannot (in generic) order against completion of prior instructions,
> > like clearing the interrupt enabled flags.
> >
> > Possibly you want barrier_nospec().
>
> I'm still really confused about this barrier. It just doesn't make
> sense.
>
> If an interrupt happens before the local_irq_disable()/save(), then it
> will discard any and all speculation that would be in progress to handle
> the exception.
>
> If there isn't an interrupt (or it happens after disable) it is
> irrelevant.
>
> Specifically, that serialize-IPI thing wants to ensure in-progress
> lookups are complete, and I can't find a scenario where
> local_irq_disable/enable() needs additional help vs IPIs. The moment an
> interrupt lands it kills speculation and forces things into
> program-order.
>
> Did you perhaps want something like:
>
> if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING)) {
> atomic_inc(&foo);
> smp_mb__after_atomic();
> }
>
> ...
>
> if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING)) {
> smp_mb__before_atomic();
> atomic_dec(&foo);
> }
>
> To ensure everything happens inside of the increment?
>
I need to rethink this barrier, but yes. I think that's it.
It's how it was on v4.
I have changed it because I thought it would be better this way. Well,
it was probably a mistake of my part.
> And I still think all that wrong, you really shouldn't need to wait on
> munmap().
That is something I need to better understand. I mean, before coming
with this patch, I thought exactly this: not serialize when on munmap.
But on the way I was convinced it would not work on munmap. I need to
recall why, and if it was false to assume this, re-think the whole
solution.
Best regards,
Leonardo Brás
[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
next prev parent reply other threads:[~2019-10-03 21:24 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 1:33 [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks Leonardo Bras
2019-10-03 7:11 ` Peter Zijlstra
2019-10-03 11:51 ` Peter Zijlstra
2019-10-03 20:40 ` John Hubbard
2019-10-04 11:24 ` Peter Zijlstra
2019-10-03 21:24 ` Leonardo Bras [this message]
2019-10-04 11:28 ` Peter Zijlstra
2019-10-09 18:09 ` Leonardo Bras
2019-10-05 8:35 ` Aneesh Kumar K.V
2019-10-08 14:47 ` Kirill A. Shutemov
2019-10-03 1:33 ` [PATCH v5 02/11] powerpc/mm: Adds counting method " Leonardo Bras
2019-10-08 15:11 ` Christopher Lameter
2019-10-08 17:13 ` Leonardo Bras
2019-10-08 17:43 ` Christopher Lameter
2019-10-08 18:02 ` Leonardo Bras
2019-10-08 18:27 ` Christopher Lameter
2019-10-03 1:33 ` [PATCH v5 03/11] mm/gup: Applies counting method to monitor gup_pgd_range Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 04/11] powerpc/mce_power: Applies counting method to monitor lockless pgtbl walks Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 05/11] powerpc/perf: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 07/11] powerpc/kvm/e500: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 09/11] powerpc/kvm/book3s_64: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 10/11] mm/Kconfig: Adds config option to track lockless pagetable walks Leonardo Bras
2019-10-03 2:08 ` Qian Cai
2019-10-03 19:04 ` Leonardo Bras
2019-10-03 19:08 ` Leonardo Bras
2019-10-03 7:44 ` Peter Zijlstra
2019-10-03 20:40 ` Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-10-03 7:29 ` [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks Peter Zijlstra
2019-10-03 20:36 ` Leonardo Bras
2019-10-03 20:49 ` John Hubbard
2019-10-03 21:38 ` Leonardo Bras
2019-10-04 11:42 ` Peter Zijlstra
2019-10-04 12:57 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c46ba8cec981ad28383bb7b23161fb83ccda4a60.camel@linux.ibm.com \
--to=leonardo@linux.ibm.com \
--cc=aarcange@redhat.com \
--cc=adobriyan@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=allison@lohutok.net \
--cc=aneesh.kumar@linux.ibm.com \
--cc=arbab@linux.ibm.com \
--cc=arnd@arndb.de \
--cc=aryabinin@virtuozzo.com \
--cc=b.zolnierkie@samsung.com \
--cc=brouer@redhat.com \
--cc=christian.brauner@ubuntu.com \
--cc=cl@linux.com \
--cc=dan.j.williams@intel.com \
--cc=dave@stgolabs.net \
--cc=elena.reshetova@intel.com \
--cc=gregkh@linuxfoundation.org \
--cc=guro@fb.com \
--cc=ira.weiny@intel.com \
--cc=jannh@google.com \
--cc=jgg@ziepe.ca \
--cc=jglisse@redhat.com \
--cc=jhubbard@nvidia.com \
--cc=jrdr.linux@gmail.com \
--cc=keith.busch@intel.com \
--cc=kirill.shutemov@linux.intel.com \
--cc=kvm-ppc@vger.kernel.org \
--cc=ldv@altlinux.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=logang@deltatee.com \
--cc=mahesh@linux.vnet.ibm.com \
--cc=mathieu.desnoyers@efficios.com \
--cc=mhocko@suse.com \
--cc=mingo@kernel.org \
--cc=npiggin@gmail.com \
--cc=paulus@samba.org \
--cc=peterz@infradead.org \
--cc=rcampbell@nvidia.com \
--cc=rppt@linux.ibm.com \
--cc=santosh@fossix.org \
--cc=songliubraving@fb.com \
--cc=tglx@linutronix.de \
--cc=vbabka@suse.cz \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).