arm64: uprobes: Optimize cache flushes for xol slot

The profiling of single-thread selftests bench reveals a bottlenect in
caches_clean_inval_pou() on ARM64. On my local testing machine, this
function takes approximately 34% of CPU cycles for trig-uprobe-nop and
trig-uprobe-push.

This patch add a check to avoid unnecessary cache flush when writing
instruction to the xol slot. If the instruction is same with the
existing instruction in slot, there is no need to synchronize D/I cache.
Since xol slot allocation and updates occur on the hot path of uprobe
handling, The upstream kernel running on Kunpeng916 (Hi1616), 4 NUMA
nodes, 64 cores@ 2.4GHz reveals this optimization has obvious gain for
nop and push testcases.

Before (next-20240918)
----------------------
uprobe-nop      ( 1 cpus):    0.418 ± 0.001M/s  (  0.418M/s/cpu)
uprobe-push     ( 1 cpus):    0.411 ± 0.005M/s  (  0.411M/s/cpu)
uprobe-ret      ( 1 cpus):    2.052 ± 0.002M/s  (  2.052M/s/cpu)
uretprobe-nop   ( 1 cpus):    0.350 ± 0.000M/s  (  0.350M/s/cpu)
uretprobe-push  ( 1 cpus):    0.353 ± 0.000M/s  (  0.353M/s/cpu)
uretprobe-ret   ( 1 cpus):    1.074 ± 0.001M/s  (  1.074M/s/cpu)

After
-----
uprobe-nop      ( 1 cpus):    0.926 ± 0.000M/s  (  0.926M/s/cpu)
uprobe-push     ( 1 cpus):    0.910 ± 0.001M/s  (  0.910M/s/cpu)
uprobe-ret      ( 1 cpus):    2.056 ± 0.001M/s  (  2.056M/s/cpu)
uretprobe-nop   ( 1 cpus):    0.653 ± 0.001M/s  (  0.653M/s/cpu)
uretprobe-push  ( 1 cpus):    0.645 ± 0.000M/s  (  0.645M/s/cpu)
uretprobe-ret   ( 1 cpus):    1.093 ± 0.001M/s  (  1.093M/s/cpu)

Signed-off-by: Liao Chang <liaochang1@huawei.com>
Link: https://lore.kernel.org/r/20240919121719.2148361-1-liaochang1@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/probes/uprobes.c b/arch/arm64/kernel/probes/uprobes.c
index d49aef2657cdf74311bbbb9f6e4415b92c114b3c..ba61651db4b5551c6ac1e5a9a0113a3be7caada4 100644 (file)
--- a/arch/arm64/kernel/probes/uprobes.c
+++ b/arch/arm64/kernel/probes/uprobes.c
@@ -17,12 +17,20 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
        void *xol_page_kaddr = kmap_atomic(page);
        void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK);
 
+       /*
+        * Initial cache maintenance of the xol page done via set_pte_at().
+        * Subsequent CMOs only needed if the xol slot changes.
+        */
+       if (!memcmp(dst, src, len))
+               goto done;
+
        /* Initialize the slot */
        memcpy(dst, src, len);
 
        /* flush caches (dcache/icache) */
        sync_icache_aliases((unsigned long)dst, (unsigned long)dst + len);
 
+done:
        kunmap_atomic(xol_page_kaddr);
 }