[PATCH v2] riscv: Add support for BATCHED_UNMAP_TLB_FLUSH

[Nam Cao]

On Mon,  8 Jan 2024 20:36:40 +0100 Alexandre Ghiti <alexghiti at rivosinc.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 at rivosinc.com>
> Reviewed-by: Jisheng Zhang <jszhang at kernel.org>
> Tested-by: Jisheng Zhang <jszhang at kernel.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 at linutronix.de>

Best regards,
Nam