Re: [PATCH] riscv: Add support for BATCHED_UNMAP_TLB_FLUSH

From: Alexandre Ghiti
Date: Mon Jan 08 2024 - 09:27:51 EST

Hi Jisheng,

On 06/01/2024 15:05, Jisheng Zhang wrote:
On Sat, Jan 06, 2024 at 09:47:04PM +0800, Jisheng Zhang wrote:
On Fri, Jan 05, 2024 at 02:36:44PM +0100, Alexandre Ghiti wrote:
On Thu, Jan 4, 2024 at 6:42 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
Hi Jisheng,

On Wed, Jan 3, 2024 at 12:10 PM Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
On Tue, Jan 02, 2024 at 03:18:51PM +0100, Alexandre Ghiti wrote:
Allow to defer the flushing of the TLB when unmapping pges, 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") shows
good performance improvement and perf reports an important decrease in
time spent flushing the tlb (results come from qemu):
Hi Alex,

I tried this micro benchmark with your patch on T-HEAD TH1520 platform, I
didn't see any performance improvement for the micro benchmark. Per
myunderstanding, the micro benchmark is special case for arm64 because
in a normal tlb flush flow, below sequence is necessary:

tlbi
dsb


while with BATCHED_UNMAP_TLB_FLUSH, the arm64 just does 'tlbi', leaving
the dsb to the arch_tlbbatch_flush(). So the final result is

several 'tlbi + dsb' sequence VS. several 'tlbi' instructions + only one dsb
The performance improvement comes from the unnecessary dsb eliminations.
Some batching should take place, and with this patch, we only send one
"full" sfence.vma instead of a "local" sfence.vma for each page, it
seems weird that you don't see any improvement, I would have thought
that one "full" sfence.vma would be better.

Do you have suitable benchmark(s) for BATCHED_UNMAP_TLB_FLUSH on riscv?
Can you give the following benchmark a try? I simply created threads
and dispatched them on all the cpus to force IPI usage, that should be
way better if the batching of the first ubenchmark is not enough to
exacerbate performance improvements, let me know and thanks for your
tests!

#define _GNU_SOURCE
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <sched.h>
#include <time.h>

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()
{
#define SIZE (1 * 1024 * 1024)
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 < 500 /* 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;

}

So I removed the dust from my unmatched and ran the benchmarks I proposed:

Without this patch:
* benchmark from commit 43b3dfdd0455 (4 runs) : ~20.3s
* same benchmark with threads (4 runs) : ~27.4s

With this patch:
* benchmark from commit 43b3dfdd0455 (4 runs) : ~17.9s
* same benchmark with threads (4 runs) : ~18.1s

So a small improvement for the single thread benchmark, but it depends
on the number of pages that get flushed, so to me that's not
applicable for the general case. For the same benchmark with multiple
threads, that's ~34% improvement. I'll add those numbers to the v2,
and JIsheng if you can provide some too, I'll add them too!
Hi Alex,

the threaded version show ~78% improvement! impressive!
One more thing when you cook v2: it's better to patch the riscv entry
in Documentation/features/vm/TLB/arch-support.txt

So for the patch:

Reviewed-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
Tested-by: Jisheng Zhang <jszhang@xxxxxxxxxx>

Thanks


Thanks for your tests and review, and the missing documentation update!

Alex


Thanks,

Alex

Thanks

Before this patch:

real 2m1.135s
user 0m0.980s
sys 2m0.096s

4.83% batch_tlb [kernel.kallsyms] [k] __flush_tlb_range

After this patch:

real 1m0.543s
user 0m1.059s
sys 0m59.489s

0.14% batch_tlb [kernel.kallsyms] [k] __flush_tlb_range

Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
---
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/tlbbatch.h | 15 +++++++
arch/riscv/include/asm/tlbflush.h | 10 +++++
arch/riscv/mm/tlbflush.c | 71 ++++++++++++++++++++++---------
4 files changed, 77 insertions(+), 20 deletions(-)
create mode 100644 arch/riscv/include/asm/tlbbatch.h

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 7603bd8ab333..aa07bd43b138 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -53,6 +53,7 @@ config RISCV
select ARCH_USE_MEMTEST
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_USES_CFI_TRAPS if CFI_CLANG
+ select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH if SMP && MMU
select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
select ARCH_WANT_FRAME_POINTERS
select ARCH_WANT_GENERAL_HUGETLB if !RISCV_ISA_SVNAPOT
diff --git a/arch/riscv/include/asm/tlbbatch.h b/arch/riscv/include/asm/tlbbatch.h
new file mode 100644
index 000000000000..46014f70b9da
--- /dev/null
+++ b/arch/riscv/include/asm/tlbbatch.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2023 Rivos Inc.
+ */
+
+#ifndef _ASM_RISCV_TLBBATCH_H
+#define _ASM_RISCV_TLBBATCH_H
+
+#include <linux/cpumask.h>
+
+struct arch_tlbflush_unmap_batch {
+ struct cpumask cpumask;
+};
+
+#endif /* _ASM_RISCV_TLBBATCH_H */
diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
index 8f3418c5f172..f0b731ccc0c2 100644
--- a/arch/riscv/include/asm/tlbflush.h
+++ b/arch/riscv/include/asm/tlbflush.h
@@ -46,6 +46,16 @@ void flush_tlb_kernel_range(unsigned long start, unsigned long end);
void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
unsigned long end);
#endif
+
+#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
+bool arch_tlbbatch_should_defer(struct mm_struct *mm);
+void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
+ struct mm_struct *mm,
+ unsigned long uaddr);
+void arch_flush_tlb_batched_pending(struct mm_struct *mm);
+void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
+#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
+
#else /* CONFIG_SMP && CONFIG_MMU */

#define flush_tlb_all() local_flush_tlb_all()
diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index e6659d7368b3..bb623bca0a7d 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -93,29 +93,23 @@ static void __ipi_flush_tlb_range_asid(void *info)
local_flush_tlb_range_asid(d->start, d->size, d->stride, d->asid);
}

-static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
- unsigned long size, unsigned long stride)
+static void __flush_tlb_range(struct cpumask *cmask, unsigned long asid,
+ unsigned long start, unsigned long size,
+ unsigned long stride)
{
struct flush_tlb_range_data ftd;
- const struct cpumask *cmask;
- unsigned long asid = FLUSH_TLB_NO_ASID;
bool broadcast;

- if (mm) {
- unsigned int cpuid;
+ if (cpumask_empty(cmask))
+ return;

- cmask = mm_cpumask(mm);
- if (cpumask_empty(cmask))
- return;
+ if (cmask != cpu_online_mask) {
+ unsigned int cpuid;

cpuid = get_cpu();
/* check if the tlbflush needs to be sent to other CPUs */
broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
-
- if (static_branch_unlikely(&use_asid_allocator))
- asid = atomic_long_read(&mm->context.id) & asid_mask;
} else {
- cmask = cpu_online_mask;
broadcast = true;
}

@@ -135,25 +129,34 @@ static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
local_flush_tlb_range_asid(start, size, stride, asid);
}

- if (mm)
+ if (cmask != cpu_online_mask)
put_cpu();
}

+static inline unsigned long get_mm_asid(struct mm_struct *mm)
+{
+ return static_branch_unlikely(&use_asid_allocator) ?
+ atomic_long_read(&mm->context.id) & asid_mask : FLUSH_TLB_NO_ASID;
+}
+
void flush_tlb_mm(struct mm_struct *mm)
{
- __flush_tlb_range(mm, 0, FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
+ __flush_tlb_range(mm_cpumask(mm), get_mm_asid(mm),
+ 0, FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
}

void flush_tlb_mm_range(struct mm_struct *mm,
unsigned long start, unsigned long end,
unsigned int page_size)
{
- __flush_tlb_range(mm, start, end - start, page_size);
+ __flush_tlb_range(mm_cpumask(mm), get_mm_asid(mm),
+ start, end - start, page_size);
}

void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
{
- __flush_tlb_range(vma->vm_mm, addr, PAGE_SIZE, PAGE_SIZE);
+ __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
+ addr, PAGE_SIZE, PAGE_SIZE);
}

void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
@@ -185,18 +188,46 @@ void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
}
}

- __flush_tlb_range(vma->vm_mm, start, end - start, stride_size);
+ __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
+ start, end - start, stride_size);
}

void flush_tlb_kernel_range(unsigned long start, unsigned long end)
{
- __flush_tlb_range(NULL, start, end - start, PAGE_SIZE);
+ __flush_tlb_range((struct cpumask *)cpu_online_mask, FLUSH_TLB_NO_ASID,
+ start, end - start, PAGE_SIZE);
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
unsigned long end)
{
- __flush_tlb_range(vma->vm_mm, start, end - start, PMD_SIZE);
+ __flush_tlb_range(mm_cpumask(vma->vm_mm), get_mm_asid(vma->vm_mm),
+ start, end - start, PMD_SIZE);
}
#endif
+
+#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
+bool arch_tlbbatch_should_defer(struct mm_struct *mm)
+{
+ return true;
+}
+
+void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
+ struct mm_struct *mm,
+ unsigned long uaddr)
+{
+ cpumask_or(&batch->cpumask, &batch->cpumask, mm_cpumask(mm));
+}
+
+void arch_flush_tlb_batched_pending(struct mm_struct *mm)
+{
+ flush_tlb_mm(mm);
+}
+
+void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
+{
+ __flush_tlb_range(&batch->cpumask, FLUSH_TLB_NO_ASID, 0,
+ FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
+}
+#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
--
2.39.2


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