Re: [PATCH v3 4/4] arm64: support batched/deferred tlb shootdown during page reclamation

On Wed, Sep 21, 2022 at 1:50 PM Barry Song [off-list ref] wrote:

On Tue, Sep 20, 2022 at 8:45 PM Anshuman Khandual
[off-list ref] wrote:

quoted

On 9/20/22 09:09, Barry Song wrote:

quoted

On Tue, Sep 20, 2022 at 3:00 PM Anshuman Khandual
[off-list ref] wrote:

quoted

On 8/22/22 13:51, Yicong Yang wrote:

quoted

+static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
+{
+     return true;
+}

This needs to be conditional on systems, where there will be performance
improvements, and should not just be enabled all the time on all systems.
num_online_cpus() > X, which does not hold any cpu hotplug lock would be
a good metric ?

for a small system, i don't see how this patch will help, e.g. cpus <= 4;
so we can actually disable tlb-batch on small systems.

Do not subscribe ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH based on NR_CPUS ?
That might not help much as the default value is 256 for NR_CPUS.

OR

arch_tlbbatch_should_defer() checks on

1. online cpus                  (dont enable batched TLB if <= X)
2. ARM64_WORKAROUND_REPEAT_TLBI (dont enable batched TLB)

quoted

just need to check if we will have any race condition since hotplug will
make the condition true and false dynamically.

If should_defer_flush() evaluate to be false, then ptep_clear_flush()
clears and flushes the entry right away. This should not race with other
queued up TLBI requests, which will be flushed separately. Wondering how
there can be a race here !

Right. How about we make something as below?

static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
{
    /* for a small system very small number of CPUs, TLB shootdown is cheap */
    if (num_online_cpus() <= 4 ||
unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
          return false;

#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
    if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
         return false;
#endif

    return true;
}

sorry, i mean

static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
{
    /* for a small system very small number of CPUs, TLB shootdown is cheap */
    if (num_online_cpus() <= 4)
            return false;

#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
    if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
            return false;
#endif

    return true;
}

Thanks
Barry