From 8a237adf213d73671992266eff7437f1b9f40567 Mon Sep 17 00:00:00 2001 From: Marcelo Tosatti Date: Tue, 27 Jun 2023 17:08:15 -0300 Subject: 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 Acked-by: Frederic Weisbecker Message-Id: Signed-off-by: Christian Brauner --- fs/buffer.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/fs/buffer.c b/fs/buffer.c index bd091329026c..3aa2380a75e1 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -49,6 +49,7 @@ #include #include #include +#include #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]); -- cgit v1.2.3