[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZJtBrybavtb1x45V@tpad>
Date: Tue, 27 Jun 2023 17:08:15 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
Frederic Weisbecker <frederic@...nel.org>,
Dave Chinner <david@...morbit.com>,
Valentin Schneider <vschneid@...hat.com>,
Leonardo Bras <leobras@...hat.com>,
Yair Podemsky <ypodemsk@...hat.com>, P J P <ppandit@...hat.com>
Subject: [PATCH] fs/buffer.c: disable per-CPU buffer_head cache for isolated
CPUs
For certain types of applications (for example PLC software or
RAN processing), upon occurrence of an event, it is necessary to
complete a certain task in a maximum amount of time (deadline).
One way to express this requirement is with a pair of numbers,
deadline time and execution time, where:
* deadline time: length of time between event and deadline.
* execution time: length of time it takes for processing of event
to occur on a particular hardware platform
(uninterrupted).
The particular values depend on use-case. For the case
where the realtime application executes in a virtualized
guest, an IPI which must be serviced in the host will cause
the following sequence of events:
1) VM-exit
2) execution of IPI (and function call)
3) VM-entry
Which causes an excess of 50us latency as observed by cyclictest
(this violates the latency requirement of vRAN application with 1ms TTI,
for example).
invalidate_bh_lrus calls an IPI on each CPU that has non empty
per-CPU cache:
on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);
The performance when using the per-CPU LRU cache is as follows:
42 ns per __find_get_block
68 ns per __find_get_block_slow
Given that the main use cases for latency sensitive applications
do not involve block I/O (data necessary for program operation is
locked in RAM), disable per-CPU buffer_head caches for isolated CPUs.
Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
diff --git a/fs/buffer.c b/fs/buffer.c
index a7fc561758b1..49e9160ce100 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -49,6 +49,7 @@
#include <trace/events/block.h>
#include <linux/fscrypt.h>
#include <linux/fsverity.h>
+#include <linux/sched/isolation.h>
#include "internal.h"
@@ -1289,7 +1290,7 @@ static void bh_lru_install(struct buffer_head *bh)
* failing page migration.
* Skip putting upcoming bh into bh_lru until migration is done.
*/
- if (lru_cache_disabled()) {
+ if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) {
bh_lru_unlock();
return;
}
@@ -1319,6 +1320,10 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
check_irqs_on();
bh_lru_lock();
+ if (cpu_is_isolated(smp_processor_id())) {
+ bh_lru_unlock();
+ return NULL;
+ }
for (i = 0; i < BH_LRU_SIZE; i++) {
struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
Powered by blists - more mailing lists