linux-kernel - Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb

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Date:   Thu, 25 Apr 2019 21:29:30 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb_info from the stack


* Nadav Amit <namit@...are.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@...radead.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Nadav Amit <namit@...are.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