Re: [PATCH v8 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

Yicong Yang
2023-04-01T12:41:42Z

On 2023/3/30 21:45, Yicong Yang wrote:
> Hi Punit,
>
> On 2023/3/30 21:15, Punit Agrawal wrote:
>> Hi Yicong,
>>
>> Yicong Yang <yangyicong@huawei.com> writes:
>>
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> on x86, batched and deferred tlb shootdown has lead to 90%
>>> performance increase on tlb shootdown. on arm64, HW can do
>>> tlb shootdown without software IPI. But sync tlbi is still
>>> quite expensive.
>>>
>>> Even running a simplest program which requires swapout can
>>> prove this is true,
>>> #include <sys/types.h>
>>> #include <unistd.h>
>>> #include <sys/mman.h>
>>> #include <string.h>
>>>
>>> int main()
>>> {
>>> #define SIZE (1 * 1024 * 1024)
>>> volatile unsigned char *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
>>> MAP_SHARED | MAP_ANONYMOUS, -1, 0);
>>>
>>> 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);
>>> }
>>> }
>>>
>>> Perf result on snapdragon 888 with 8 cores by using zRAM
>>> as the swap block device.
>>>
>>> ~ # perf record taskset -c 4 ./a.out
>>> [ perf record: Woken up 10 times to write data ]
>>> [ perf record: Captured and wrote 2.297 MB perf.data (60084 samples) ]
>>> ~ # perf report
>>> # To display the perf.data header info, please use --header/--header-only options.
>>> # To display the perf.data header info, please use --header/--header-only options.
>>> #
>>> #
>>> # Total Lost Samples: 0
>>> #
>>> # Samples: 60K of event 'cycles'
>>> # Event count (approx.): 35706225414
>>> #
>>> # Overhead Command Shared Object Symbol
>>> # ........ ....... ................. .............................................................................
>>> #
>>> 21.07% a.out [kernel.kallsyms] [k] _raw_spin_unlock_irq
>>> 8.23% a.out [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
>>> 6.67% a.out [kernel.kallsyms] [k] filemap_map_pages
>>> 6.16% a.out [kernel.kallsyms] [k] __zram_bvec_write
>>> 5.36% a.out [kernel.kallsyms] [k] ptep_clear_flush
>>> 3.71% a.out [kernel.kallsyms] [k] _raw_spin_lock
>>> 3.49% a.out [kernel.kallsyms] [k] memset64
>>> 1.63% a.out [kernel.kallsyms] [k] clear_page
>>> 1.42% a.out [kernel.kallsyms] [k] _raw_spin_unlock
>>> 1.26% a.out [kernel.kallsyms] [k] mod_zone_state.llvm.8525150236079521930
>>> 1.23% a.out [kernel.kallsyms] [k] xas_load
>>> 1.15% a.out [kernel.kallsyms] [k] zram_slot_lock
>>>
>>> ptep_clear_flush() takes 5.36% CPU in the micro-benchmark
>>> swapping in/out a page mapped by only one process. If the
>>> page is mapped by multiple processes, typically, like more
>>> than 100 on a phone, the overhead would be much higher as
>>> we have to run tlb flush 100 times for one single page.
>>> Plus, tlb flush overhead will increase with the number
>>> of CPU cores due to the bad scalability of tlb shootdown
>>> in HW, so those ARM64 servers should expect much higher
>>> overhead.
>>>
>>> Further perf annonate shows 95% cpu time of ptep_clear_flush
>>> is actually used by the final dsb() to wait for the completion
>>> of tlb flush. This provides us a very good chance to leverage
>>> the existing batched tlb in kernel. The minimum modification
>>> is that we only send async tlbi in the first stage and we send
>>> dsb while we have to sync in the second stage.
>>>
>>> With the above simplest micro benchmark, collapsed time to
>>> finish the program decreases around 5%.
>>>
>>> Typical collapsed time w/o patch:
>>> ~ # time taskset -c 4 ./a.out
>>> 0.21user 14.34system 0:14.69elapsed
>>> w/ patch:
>>> ~ # time taskset -c 4 ./a.out
>>> 0.22user 13.45system 0:13.80elapsed
>>>
>>> Also, Yicong Yang added the following observation.
>>> Tested with benchmark in the commit on Kunpeng920 arm64 server,
>>> observed an improvement around 12.5% with command
>>> `time ./swap_bench`.
>>> w/o w/
>>> real 0m13.460s 0m11.771s
>>> user 0m0.248s 0m0.279s
>>> sys 0m12.039s 0m11.458s
>>>
>>> Originally it's noticed a 16.99% overhead of ptep_clear_flush()
>>> which has been eliminated by this patch:
>>>
>>> [root@localhost yang]# perf record -- ./swap_bench && perf report
>>> [...]
>>> 16.99% swap_bench [kernel.kallsyms] [k] ptep_clear_flush
>>>
>>> It is tested on 4,8,128 CPU platforms and shows to be beneficial on
>>> large systems but may not have improvement on small systems like on
>>> a 4 CPU platform. So make ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH depends
>>> on CONFIG_EXPERT for this stage and make this disabled on systems
>>> with less than 8 CPUs. User can modify this threshold according to
>>> their own platforms by CONFIG_NR_CPUS_FOR_BATCHED_TLB.
>>
>> The commit log and the patch disagree on the name of the config option
>> (CONFIG_NR_CPUS_FOR_BATCHED_TLB vs CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB).
>>
>
> ah yes, it's a typo and I'll fix it.
>
>> But more importantly, I was wondering why this posting doesn't address
>> Catalin's feedback [a] about using a runtime tunable. Maybe I missed the
>> follow-up discussion.
>>
>

So I used below patch based on this to provide a knob /proc/sys/vm/batched_tlb_enabled
for turning on/off the batched TLB. But wondering flush.c is the best place for putting
this, any comments?

Thanks.

diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
index 41a763cf8c1b..2b2c69c23b47 100644
--- a/arch/arm64/include/asm/tlbflush.h
+++ b/arch/arm64/include/asm/tlbflush.h
@@ -280,6 +280,8 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,

#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH

+extern struct static_key_false batched_tlb_enabled;
+
static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
{
/*
@@ -289,7 +291,7 @@ static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
* a threshold for enabling this to avoid potential side effects on
* these platforms.
*/
- if (num_online_cpus() < CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
+ if (!static_branch_unlikely(&batched_tlb_enabled))
return false;

/*
diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
index 5f9379b3c8c8..ce3bc32523f7 100644
--- a/arch/arm64/mm/flush.c
+++ b/arch/arm64/mm/flush.c
@@ -7,8 +7,10 @@
*/

#include <linux/export.h>
+#include <linux/jump_label.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
+#include <linux/sysctl.h>

#include <asm/cacheflush.h>
#include <asm/cache.h>
@@ -107,3 +109,53 @@ void arch_invalidate_pmem(void *addr, size_t size)
}
EXPORT_SYMBOL_GPL(arch_invalidate_pmem);
#endif
+
+#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
+
+DEFINE_STATIC_KEY_FALSE(batched_tlb_enabled);
+
+int batched_tlb_enabled_handler(struct ctl_table *table, int write,
+ void *buffer, size_t *lenp, loff_t *ppos)
+{
+ unsigned int enabled = static_branch_unlikely(&batched_tlb_enabled);
+ struct ctl_table t;
+ int err;
+
+ if (write && !capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
+ t = *table;
+ t.data = &enabled;
+ err = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
+ if (!err && write) {
+ if (enabled)
+ static_branch_enable(&batched_tlb_enabled);
+ else
+ static_branch_disable(&batched_tlb_enabled);
+ }
+
+ return err;
+}
+
+static struct ctl_table batched_tlb_sysctls[] = {
+ {
+ .procname = "batched_tlb_enabled",
+ .data = NULL,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = batched_tlb_enabled_handler,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ },
+ {}
+};
+
+static int __init batched_tlb_sysctls_init(void)
+{
+ register_sysctl_init("vm", batched_tlb_sysctls);
+
+ return 0;
+}
+late_initcall(batched_tlb_sysctls_init);
+
+#endif