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

* 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()?

> +
> + info->start = start;
> + info->end = end;
> + info->mm = mm;
> + info->stride_shift = stride_shift;
> + info->freed_tables = freed_tables;
> + info->new_tlb_gen = new_tlb_gen;

This pattern is really asking for a bit more tabulation:

info->start = start;
info->end = end;
info->mm = mm;
info->stride_shift = stride_shift;
info->freed_tables = freed_tables;
info->new_tlb_gen = new_tlb_gen;

> +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?

> + if ((end == TLB_FLUSH_ALL) ||
> + ((end - start) >> stride_shift) > tlb_single_page_flush_ceiling) {
> + start = 0UL;

Any reason why we cannot use the '0' numeric literal here?

> + struct flush_tlb_info *info;
> +
> + preempt_disable();
> +
> + info = get_flush_tlb_info(NULL, start, end, 0, false, 0);
> +
> + info = this_cpu_ptr(&flush_tlb_info);
> + info->start = start;
> + info->end = end;
> +
> + on_each_cpu(do_kernel_range_flush, info, 1);
> +
> + put_flush_tlb_info();
> +
> + preempt_enable();

> }
> }
>
> void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> {
> - struct flush_tlb_info info = {
> + static const struct flush_tlb_info info = {
> .mm = NULL,
> .start = 0UL,
> .end = TLB_FLUSH_ALL,

Could you please move this now .data global variable just before the
function, to make it more apparent what's going on?

Thanks,

Ingo

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