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

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Date:   Tue, 23 Apr 2019 16:56:54 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] x86/mm/tlb: Remove flush_tlb_info from the stack

> On Apr 23, 2019, at 9:50 AM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Tue, Apr 23, 2019 at 12:12 AM Nadav Amit <namit@...are.com> wrote:
>> Remove flush_tlb_info variables from 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):
> 
> Can you add a memset(,,,. 0, sizeof(struct flush_tlb_info)) everywhere
> you grab it?  Or, even better, perhaps do something like:
> 
> static inline struct flush_tlb_info *get_flush_tlb_info(void)
> {
>  /* check reentrancy, make sure that we use smp_processor_id() or
> otherwise assert that we're bound to a single CPU. */
>  struct flush_tlb_info *ptr = this_cpu_ptr(...);
>  memset(ptr, 0, sizeof(*ptr));
>  return ptr;
> }
> 
> static inline void put_flush_tlb_info(void)
> {
> /* finish checking reentrancy. */
> }

I’ll check if the compiler is smart enough to avoid redundant assignments,
and if it is not, I’ll just give all the struct arguments to
get_flush_tlb_info() instead of memset() if you don’t mind.

I also want to give a try for parallelizing the remote and local
invocations, which really annoys me every time I look at the code. Please
let me know if there is any “big reason” that I am missing for it not to be
done before. I just hope all the paravirt stuff will not make it too ugly.