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>
#include <trace/events/block.h>
#include <linux/fscrypt.h>
#include <linux/fsverity.h>
+#include <linux/sched/isolation.h>
#include "internal.h"
* 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;
}
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]);