Messages in this thread Patch in this message | | | Date | Tue, 27 Jun 2023 17:08:15 -0300 | From | Marcelo Tosatti <> | Subject | [PATCH] 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>
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]);
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