From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH v6 10/11] powerpc/mm: Adds counting method to track lockless pagetable walks References: <20200206030900.147032-1-leonardo@linux.ibm.com> <20200206030900.147032-11-leonardo@linux.ibm.com> From: Christophe Leroy Message-ID: Date: Thu, 6 Feb 2020 07:23:24 +0100 MIME-Version: 1.0 In-Reply-To: <20200206030900.147032-11-leonardo@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit To: Leonardo Bras , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Arnd Bergmann , Andrew Morton , "Aneesh Kumar K.V" , Nicholas Piggin , Steven Price , Robin Murphy , Mahesh Salgaonkar , Balbir Singh , Reza Arbab , Thomas Gleixner , Allison Randal , Greg Kroah-Hartman , Mike Rapoport , Michal Suchanek Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org List-ID: Message-ID: <20200206062324.fkrZXZ-cxIvlpaaV1CkO0m4c2M-seNkns_4-e2mlDmY@z> Le 06/02/2020 à 04:08, Leonardo Bras a écrit : > Implements an additional feature to track lockless pagetable walks, > using a per-cpu counter: lockless_pgtbl_walk_counter. > > Before a lockless pagetable walk, preemption is disabled and the > current cpu's counter is increased. > When the lockless pagetable walk finishes, the current cpu counter > is decreased and the preemption is enabled. > > With that, it's possible to know in which cpus are happening lockless > pagetable walks, and optimize serialize_against_pte_lookup(). > > Implementation notes: > - Every counter can be changed only by it's CPU > - It makes use of the original memory barrier in the functions > - Any counter can be read by any CPU > > Due to not locking nor using atomic variables, the impact on the > lockless pagetable walk is intended to be minimum. atomic variables have a lot less impact than preempt_enable/disable. preemt_disable forces a re-scheduling, it really has impact. Why not use atomic variables instead ? Christophe > > Signed-off-by: Leonardo Bras > --- > arch/powerpc/mm/book3s64/pgtable.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c > index 535613030363..bb138b628f86 100644 > --- a/arch/powerpc/mm/book3s64/pgtable.c > +++ b/arch/powerpc/mm/book3s64/pgtable.c > @@ -83,6 +83,7 @@ static void do_nothing(void *unused) > > } > > +static DEFINE_PER_CPU(int, lockless_pgtbl_walk_counter); > /* > * Serialize against find_current_mm_pte which does lock-less > * lookup in page tables with local interrupts disabled. For huge pages > @@ -120,6 +121,15 @@ unsigned long __begin_lockless_pgtbl_walk(bool disable_irq) > if (disable_irq) > local_irq_save(irq_mask); > > + /* > + * Counts this instance of lockless pagetable walk for this cpu. > + * Disables preempt to make sure there is no cpu change between > + * begin/end lockless pagetable walk, so that percpu counting > + * works fine. > + */ > + preempt_disable(); > + (*this_cpu_ptr(&lockless_pgtbl_walk_counter))++; > + > /* > * This memory barrier pairs with any code that is either trying to > * delete page tables, or split huge pages. Without this barrier, > @@ -158,6 +168,14 @@ inline void __end_lockless_pgtbl_walk(unsigned long irq_mask, bool enable_irq) > */ > smp_mb(); > > + /* > + * Removes this instance of lockless pagetable walk for this cpu. > + * Enables preempt only after end lockless pagetable walk, > + * so that percpu counting works fine. > + */ > + (*this_cpu_ptr(&lockless_pgtbl_walk_counter))--; > + preempt_enable(); > + > /* > * Interrupts must be disabled during the lockless page table walk. > * That's because the deleting or splitting involves flushing TLBs, >