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

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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>
This commit is contained in:
Marcelo Tosatti 2023-06-27 17:08:15 -03:00 committed by Christian Brauner
parent d80a8f1b58
commit 8a237adf21
1 changed files with 6 additions and 1 deletions
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7
fs/buffer.c
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@ -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"
@ -1352,7 +1353,7 @@ 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;
}
@ -1382,6 +1383,10 @@ 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]);