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

From: Nadav Amit
Date: Thu Apr 25 2019 - 14:14:02 EST

> On Apr 23, 2019, at 10:23 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Tue, Apr 23, 2019 at 9:56 AM Nadav Amit <namit@xxxxxxxxxx> wrote:
>>> On Apr 23, 2019, at 9:50 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> On Tue, Apr 23, 2019 at 12:12 AM Nadav Amit <namit@xxxxxxxxxx> wrote:
>>;;sdata=5uXX95CD%2FAI5ZSJON%2BjCAKQ88sAwZgme6Az0ZHpEGZo%3D&amp;reserved=0 >> 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. */
>>> }
>> Iâll check if the compiler is smart enough to avoid redundant assignments,
>> and if it is not, Iâll just give all the struct arguments to
>> get_flush_tlb_info() instead of memset() if you donât mind.
> Sounds good.
>> 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.
> Yes please!

I have written some patches and they do provide a considerable performance
improvement of (>10%) for remote TLB flushes. There are still some issues
that need to be resolved, specifically a small slowdown for local TLB
flushes (~15ns).

Anyhow, based on my past experience, I will do this change in a separate
patch-set after the flush_tlb_info off-stack patch makes it through.