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Merge: fs/buffer.c: disable per-CPU buffer_head cache for isolated CPUs 

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/3007

commit 8a237adf213d73671992266eff7437f1b9f40567
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2158709
Testing: tested by verifying the function call IPI is not
initiated to 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@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Message-Id: <ZJtBrybavtb1x45V@tpad>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Approved-by: Ming Lei <ming.lei@redhat.com>
Approved-by: Chris von Recklinghausen <crecklin@redhat.com>

Signed-off-by: Scott Weaver <scweaver@redhat.com>
This commit is contained in:
Scott Weaver 2023-10-17 09:01:45 -04:00
parent c007b2ed95 1bb045a419
commit bc0de7ef8f
1 changed files with 6 additions and 1 deletions
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7
fs/buffer.c
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@ -48,6 +48,7 @@
#include <linux/sched/mm.h>
#include <trace/events/block.h>
#include <linux/fscrypt.h>
#include <linux/sched/isolation.h>
#include "internal.h"
@ -1240,7 +1241,7 @@ 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;
}
@ -1270,6 +1271,10 @@ 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]);