linux-kernel - Re: [PATCH v2] riscv: Add support for BATCHED_UNMAP_TLB

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Date: Mon, 8 Jan 2024 21:44:48 +0100
From: Nam Cao <namcao@...utronix.de>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, Jisheng
 Zhang <jszhang@...nel.org>
Subject: Re: [PATCH v2] riscv: Add support for BATCHED_UNMAP_TLB_FLUSH

On Mon, 8 Jan 2024 21:43:46 +0100 Nam Cao <namcao@...utronix.de> wrote:

> On Mon,  8 Jan 2024 20:36:40 +0100 Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> > Allow to defer the flushing of the TLB when unmapping pages, which allows
> > to reduce the numbers of IPI and the number of sfence.vma.
> > 
> > The ubenchmarch used in commit 43b3dfdd0455 ("arm64: support
> > batched/deferred tlb shootdown during page reclamation/migration") that
> > was multithreaded to force the usage of IPI shows good performance
> > improvement on all platforms:
> > 
> > * Unmatched: ~34%
> > * TH1520   : ~78%
> > * Qemu     : ~81%
> > 
> > In addition, perf on qemu reports an important decrease in time spent
> > dealing with IPIs:
> > 
> > Before:  68.17%  main     [kernel.kallsyms]            [k] __sbi_rfence_v02_call
> > After :   8.64%  main     [kernel.kallsyms]            [k] __sbi_rfence_v02_call
> > 
> > * Benchmark:
> > 
> > int stick_this_thread_to_core(int core_id) {
> >         int num_cores = sysconf(_SC_NPROCESSORS_ONLN);
> >         if (core_id < 0 || core_id >= num_cores)
> >            return EINVAL;
> > 
> >         cpu_set_t cpuset;
> >         CPU_ZERO(&cpuset);
> >         CPU_SET(core_id, &cpuset);
> > 
> >         pthread_t current_thread = pthread_self();
> >         return pthread_setaffinity_np(current_thread,
> > sizeof(cpu_set_t), &cpuset);
> > }
> > 
> > static void *fn_thread (void *p_data)
> > {
> >         int ret;
> >         pthread_t thread;
> > 
> >         stick_this_thread_to_core((int)p_data);
> > 
> >         while (1) {
> >                 sleep(1);
> >         }
> > 
> >         return NULL;
> > }
> > 
> > int main()
> > {
> >         volatile unsigned char *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> >                                          MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> >         pthread_t threads[4];
> >         int ret;
> > 
> >         for (int i = 0; i < 4; ++i) {
> >                 ret = pthread_create(&threads[i], NULL, fn_thread, (void *)i);
> >                 if (ret)
> >                 {
> >                         printf("%s", strerror (ret));
> >                 }
> >         }
> > 
> >         memset(p, 0x88, SIZE);
> > 
> >         for (int k = 0; k < 10000; k++) {
> >                 /* swap in */
> >                 for (int i = 0; i < SIZE; i += 4096) {
> >                         (void)p[i];
> >                 }
> > 
> >                 /* swap out */
> >                 madvise(p, SIZE, MADV_PAGEOUT);
> >         }
> > 
> >         for (int i = 0; i < 4; i++)
> >         {
> >                 pthread_cancel(threads[i]);
> >         }
> > 
> >         for (int i = 0; i < 4; i++)
> >         {
> >                 pthread_join(threads[i], NULL);
> >         }
> > 
> >         return 0;
> > }
> > 
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > Reviewed-by: Jisheng Zhang <jszhang@...nel.org>
> > Tested-by: Jisheng Zhang <jszhang@...nel.org> # Tested on TH1520  
> 
> Before:
>     real 0m36.674s
>     user 0m0.173s
>     sys  0m36.493s
> After:
>     real 0m18.016s
>     user 0m0.125s
>     sys  0m17.885s
> 
> Tested-by: Nam Cao <namcao@...utronix.de>

I forgot to mention: this is for Starfive's Visionfive 2 board.

Best regards,
Nam