[Devel] [PATCH RH8 8/8] mm/page_alloc: use sched_clock() instead of jiffies to measure latency

[Andrey Zhadchenko]

From: Andrey Ryabinin <aryabinin at virtuozzo.com>

sched_clock() (which is rdtsc() on x86) gives us more precise result
than jiffies.

Q: Why do we need greater accuracy?
A: Because if we target to, say, 10000 IOPS (per cpu) then
   1 ms memory allocation latency is too much and we need
   to achieve less alloc latency and thus measure it.

https://pmc.acronis.com/browse/VSTOR-19040

Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>

(cherry-picked from 99407f6d6f504d00aa3cd3ca87782ab32b0ec364)
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
---
 mm/page_alloc.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d3222ac..b5afa2a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4556,9 +4556,11 @@ static void __alloc_collect_stats(gfp_t gfp_mask, unsigned int order,
 {
 #ifdef CONFIG_VE
 	unsigned long flags;
+	u64 current_clock, delta;
 	int ind, cpu;
 
-	time = jiffies_to_usecs(jiffies - time) * 1000;
+	current_clock = sched_clock();
+	delta = current_clock - time;
 	if (!(gfp_mask & __GFP_RECLAIM))
 		ind = KSTAT_ALLOCSTAT_ATOMIC;
 	else
@@ -4569,12 +4571,12 @@ static void __alloc_collect_stats(gfp_t gfp_mask, unsigned int order,
 
 	local_irq_save(flags);
 	cpu = smp_processor_id();
-	KSTAT_LAT_PCPU_ADD(&kstat_glob.alloc_lat[ind], time);
+	KSTAT_LAT_PCPU_ADD(&kstat_glob.alloc_lat[ind], delta);
 
 	if (in_task()) {
-		current->alloc_lat[ind].totlat += time;
+		current->alloc_lat[ind].totlat += delta;
 		current->alloc_lat[ind].count++;
-		update_maxlat(&current->alloc_lat[ind], time, jiffies);
+		update_maxlat(&current->alloc_lat[ind], delta, current_clock);
 	}
 
 	if (!page)
@@ -4594,7 +4596,7 @@ struct page *
 	unsigned int alloc_flags = ALLOC_WMARK_LOW;
 	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
 	struct alloc_context ac = { };
-	cycles_t start;
+	u64 start;
 
 	gfp_mask &= gfp_allowed_mask;
 	alloc_mask = gfp_mask;
@@ -4606,7 +4608,7 @@ struct page *
 
 	finalise_ac(gfp_mask, &ac);
 
-	start = jiffies;
+	start = sched_clock();
 	/* First allocation attempt */
 	page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
 	if (likely(page))
-- 
1.8.3.1