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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@redhat.com> Acked-by: Frederic Weisbecker <frederic@kernel.org> Message-Id: <ZJtBrybavtb1x45V@tpad> Signed-off-by: Christian Brauner <brauner@kernel.org>
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@ -49,6 +49,7 @@
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#include <trace/events/block.h>
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#include <linux/fscrypt.h>
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#include <linux/fsverity.h>
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#include <linux/sched/isolation.h>
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#include "internal.h"
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@ -1352,7 +1353,7 @@ static void bh_lru_install(struct buffer_head *bh)
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* failing page migration.
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* Skip putting upcoming bh into bh_lru until migration is done.
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*/
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if (lru_cache_disabled()) {
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if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) {
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bh_lru_unlock();
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return;
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}
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@ -1382,6 +1383,10 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
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check_irqs_on();
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bh_lru_lock();
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if (cpu_is_isolated(smp_processor_id())) {
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bh_lru_unlock();
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return NULL;
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}
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for (i = 0; i < BH_LRU_SIZE; i++) {
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struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
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