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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: Liao Chang

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);
 }