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srcu: Automatically determine size-transition strategy at boot

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This commit adds a srcutree.convert_to_big option of zero that causes
SRCU to decide at boot whether to wait for contention (small systems) or
immediately expand to large (large systems).  A new srcutree.big_cpu_lim
(defaulting to 128) defines how many CPUs constitute a large system.

Co-developed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-01-31 11:21:30 -08:00
parent 9f2e91d94c
commit a57ffb3c6b
2 changed files with 30 additions and 3 deletions
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10
Documentation/admin-guide/kernel-parameters.txt
View File

@ -5608,6 +5608,15 @@
off: Disable mitigation and remove
performance impact to RDRAND and RDSEED
srcutree.big_cpu_lim [KNL]
Specifies the number of CPUs constituting a
large system, such that srcu_struct structures
should immediately allocate an srcu_node array.
This kernel-boot parameter defaults to 128,
but takes effect only when the low-order four
bits of srcutree.convert_to_big is equal to 3
(decide at boot).
srcutree.convert_to_big [KNL]
Specifies under what conditions an SRCU tree
srcu_struct structure will be converted to big
@ -5616,6 +5625,7 @@
0: Never.
1: At init_srcu_struct() time.
2: When rcutorture decides to.
3: Decide at boot time (default).
0x1X: Above plus if high contention.
Either way, the srcu_node tree will be sized based

23
kernel/rcu/srcutree.c
View File

@ -41,10 +41,10 @@ module_param(counter_wrap_check, ulong, 0444);
/*
* Control conversion to SRCU_SIZE_BIG:
* 0: Don't convert at all (default).
* 0: Don't convert at all.
* 1: Convert at init_srcu_struct() time.
* 2: Convert when rcutorture invokes srcu_torture_stats_print().
* 3: Decide at boot time based on system shape.
* 3: Decide at boot time based on system shape (default).
* 0x1x: Convert when excessive contention encountered.
*/
#define SRCU_SIZING_NONE 0
@ -57,9 +57,13 @@ module_param(counter_wrap_check, ulong, 0444);
#define SRCU_SIZING_IS_INIT() (SRCU_SIZING_IS(SRCU_SIZING_INIT))
#define SRCU_SIZING_IS_TORTURE() (SRCU_SIZING_IS(SRCU_SIZING_TORTURE))
#define SRCU_SIZING_IS_CONTEND() (convert_to_big & SRCU_SIZING_CONTEND)
static int convert_to_big = SRCU_SIZING_NONE;
static int convert_to_big = SRCU_SIZING_AUTO;
module_param(convert_to_big, int, 0444);
/* Number of CPUs to trigger init_srcu_struct()-time transition to big. */
static int big_cpu_lim __read_mostly = 128;
module_param(big_cpu_lim, int, 0444);
/* Contention events per jiffy to initiate transition to big. */
static int small_contention_lim __read_mostly = 100;
module_param(small_contention_lim, int, 0444);
@ -1619,6 +1623,17 @@ void __init srcu_init(void)
{
struct srcu_struct *ssp;
/* Decide on srcu_struct-size strategy. */
if (SRCU_SIZING_IS(SRCU_SIZING_AUTO)) {
if (nr_cpu_ids >= big_cpu_lim) {
convert_to_big = SRCU_SIZING_INIT; // Don't bother waiting for contention.
pr_info("%s: Setting srcu_struct sizes to big.\n", __func__);
} else {
convert_to_big = SRCU_SIZING_NONE | SRCU_SIZING_CONTEND;
pr_info("%s: Setting srcu_struct sizes based on contention.\n", __func__);
}
}
/*
* Once that is set, call_srcu() can follow the normal path and
* queue delayed work. This must follow RCU workqueues creation
@ -1629,6 +1644,8 @@ void __init srcu_init(void)
ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
work.work.entry);
list_del_init(&ssp->work.work.entry);
if (SRCU_SIZING_IS(SRCU_SIZING_INIT) && ssp->srcu_size_state == SRCU_SIZE_SMALL)
ssp->srcu_size_state = SRCU_SIZE_ALLOC;
queue_work(rcu_gp_wq, &ssp->work.work);
}
}