Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb_info from the stack

Nadav Amit
2019-04-25T21:20:32Z

> On Apr 25, 2019, at 12:48 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Nadav Amit <namit@vmware.com> wrote:
>
>>> On Apr 25, 2019, at 12:29 PM, Ingo Molnar <mingo@kernel.org> wrote:
>>>
>>>
>>> * Nadav Amit <namit@vmware.com> wrote:
>>>
>>>> Move flush_tlb_info variables off the stack. This allows to align
>>>> flush_tlb_info to cache-line and avoid potentially unnecessary cache
>>>> line movements. It also allows to have a fixed virtual-to-physical
>>>> translation of the variables, which reduces TLB misses.
>>>>
>>>> Use per-CPU struct for flush_tlb_mm_range() and
>>>> flush_tlb_kernel_range(). Add debug assertions to ensure there are
>>>> no nested TLB flushes that might overwrite the per-CPU data. For
>>>> arch_tlbbatch_flush() use a const struct.
>>>>
>>>> Results when running a microbenchmarks that performs 10^6 MADV_DONTEED
>>>> operations and touching a page, in which 3 additional threads run a
>>>> busy-wait loop (5 runs, PTI and retpolines are turned off):
>>>>
>>>> base off-stack
>>>> ---- ---------
>>>> avg (usec/op) 1.629 1.570 (-3%)
>>>> stddev 0.014 0.009
>>>>
>>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>>> Cc: Andy Lutomirski <luto@kernel.org>
>>>> Cc: Dave Hansen <dave.hansen@intel.com>
>>>> Cc: Borislav Petkov <bp@alien8.de>
>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>> Signed-off-by: Nadav Amit <namit@vmware.com>
>>>>
>>>> ---
>>>>
>>>> v1->v2:
>>>> - Initialize all flush_tlb_info fields [Andy]
>>>> ---
>>>> arch/x86/mm/tlb.c | 100 ++++++++++++++++++++++++++++++++++------------
>>>> 1 file changed, 74 insertions(+), 26 deletions(-)
>>>>
>>>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>>>> index 487b8474c01c..aac191eb2b90 100644
>>>> --- a/arch/x86/mm/tlb.c
>>>> +++ b/arch/x86/mm/tlb.c
>>>> @@ -634,7 +634,7 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
>>>> this_cpu_write(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen, mm_tlb_gen);
>>>> }
>>>>
>>>> -static void flush_tlb_func_local(void *info, enum tlb_flush_reason reason)
>>>> +static void flush_tlb_func_local(const void *info, enum tlb_flush_reason reason)
>>>> {
>>>> const struct flush_tlb_info *f = info;
>>>>
>>>> @@ -722,43 +722,81 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
>>>> */
>>>> unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;
>>>>
>>>> +static DEFINE_PER_CPU_SHARED_ALIGNED(struct flush_tlb_info, flush_tlb_info);
>>>> +
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> +static DEFINE_PER_CPU(unsigned int, flush_tlb_info_idx);
>>>> +#endif
>>>> +
>>>> +static inline struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
>>>> + unsigned long start, unsigned long end,
>>>> + unsigned int stride_shift, bool freed_tables,
>>>> + u64 new_tlb_gen)
>>>> +{
>>>> + struct flush_tlb_info *info = this_cpu_ptr(&flush_tlb_info);
>>>> +
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> + /*
>>>> + * Ensure that the following code is non-reentrant and flush_tlb_info
>>>> + * is not overwritten. This means no TLB flushing is initiated by
>>>> + * interrupt handlers and machine-check exception handlers.
>>>> + */
>>>> + BUG_ON(this_cpu_inc_return(flush_tlb_info_idx) != 1);
>>>> +#endif
>>>
>>> isn't this effectively VM_BUG_ON()?
>>
>> Not exactly. When CONFIG_DEBUG_VM is off we get
>>
>> #define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
>>
>> This will cause the build to fail since flush_tlb_info_idx is not defined in
>> when CONFIG_DEBUG_VM is off.
>
> Ugh, so VM_BUG_ON() should really be named VM_BUILD_BUG_ON()?
>
> Anyway, agreed.
>
>>>> +static inline void put_flush_tlb_info(void)
>>>> +{
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> + /* Complete reentrency prevention checks */
>>>> + barrier();
>>>> + this_cpu_dec(flush_tlb_info_idx);
>>>> +#endif
>>>
>>> In principle this_cpu_dec() should imply a compiler barrier?
>>
>> this_cpu_dec() is eventually expanded to the macro of percpu_add_op(). And
>> the inline assembly does not have a “memory” clobber, so I don’t think so.
>
> I think that's a bug and PeterZ is fixing those.

This would be quite surprising. Even atomic_dec() does not imply a compilers
barrier. I think I should leave it as is for now, and let Peter change it
later if he decides to do so. Let me know if you disagree.