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SubjectRe: v4.16+ seeing many unaligned access in dequeue_task_fair() on IA64
On Wed, Apr 04, 2018 at 06:53:10PM +0200, Peter Zijlstra wrote:
> Awesome, I'll go get it merged, even though I don't understand where it
> went wobbly.

Ingo, could you magic this in?

---
Subject: sched: Force alignment of struct util_est
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Apr 5 09:56:16 CEST 2018

For some as yet not understood reason, Tony gets unaligned access
traps on IA64 because of:

struct util_est ue = READ_ONCE(p->se.avg.util_est);

and:

WRITE_ONCE(p->se.avg.util_est, ue);

introduced by commit:

d519329f72a6 ("sched/fair: Update util_est only on util_avg updates")

Normally those two fields should end up on an 8-byte aligned location,
but UP and RANDSTRUCT can mess that up so enforce the alignment
explicitly.

Also make the alignment on sched_avg unconditional, as it is really
about data locality, not false-sharing.

With or without this patch the layout for sched_avg on a
ia64-defconfig build looks like:

$ pahole -EC sched_avg ia64-defconfig/kernel/sched/core.o
die__process_function: tag not supported (INVALID)!
struct sched_avg {
/* typedef u64 */ long long unsigned int last_update_time; /* 0 8 */
/* typedef u64 */ long long unsigned int load_sum; /* 8 8 */
/* typedef u64 */ long long unsigned int runnable_load_sum; /* 16 8 */
/* typedef u32 */ unsigned int util_sum; /* 24 4 */
/* typedef u32 */ unsigned int period_contrib; /* 28 4 */
long unsigned int load_avg; /* 32 8 */
long unsigned int runnable_load_avg; /* 40 8 */
long unsigned int util_avg; /* 48 8 */
struct util_est {
unsigned int enqueued; /* 56 4 */
unsigned int ewma; /* 60 4 */
} util_est; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */

/* size: 64, cachelines: 1, members: 9 */
};

Fixes: d519329f72a6 ("sched/fair: Update util_est only on util_avg updates")
Reported-and-Tested-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/sched.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -300,7 +300,7 @@ struct util_est {
unsigned int enqueued;
unsigned int ewma;
#define UTIL_EST_WEIGHT_SHIFT 2
-};
+} __attribute__((__aligned__(sizeof(u64))));

/*
* The load_avg/util_avg accumulates an infinite geometric series
@@ -364,7 +364,7 @@ struct sched_avg {
unsigned long runnable_load_avg;
unsigned long util_avg;
struct util_est util_est;
-};
+} ____cacheline_aligned;

struct sched_statistics {
#ifdef CONFIG_SCHEDSTATS
@@ -435,7 +435,7 @@ struct sched_entity {
* Put into separate cache line so it does not
* collide with read-mostly values above.
*/
- struct sched_avg avg ____cacheline_aligned_in_smp;
+ struct sched_avg avg;
#endif
};

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