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rcu: Allow rcu_nocbs= to specify all CPUs

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Currently, the rcu_nocbs= kernel boot parameter requires that a specific
list of CPUs be specified, and has no way to say "all of them".
As noted by user RavFX in a comment to Phoronix topic 1002538, this
is an inconvenient side effect of the removal of the RCU_NOCB_CPU_ALL
Kconfig option.  This commit therefore enables the rcu_nocbs= kernel boot
parameter to be given the string "all", as in "rcu_nocbs=all" to specify
that all CPUs on the system are to have their RCU callbacks offloaded.

Another approach would be to make cpulist_parse() check for "all", but
there are uses of cpulist_parse() that do other checking, which could
conflict with an "all".  This commit therefore focuses on the specific
use of cpulist_parse() in rcu_nocb_setup().

Just a note to other people who would like changes to Linux-kernel RCU:
If you send your requests to me directly, they might get fixed somewhat
faster.  RavFX's comment was posted on January 22, 2018 and I first saw
it on March 5, 2019.  And the only reason that I found it -at- -all- was
that I was looking for projects using RCU, and my search engine showed
me that Phoronix comment quite by accident.  Your choice, though!  ;-)

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-03-05 15:28:19 -08:00
parent b2eb85b49a
commit da8739f23f
2 changed files with 7 additions and 2 deletions
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4
Documentation/admin-guide/kernel-parameters.txt
View File

@ -3623,7 +3623,9 @@
see CONFIG_RAS_CEC help text.
rcu_nocbs= [KNL]
The argument is a cpu list, as described above.
The argument is a cpu list, as described above,
except that the string "all" can be used to
specify every CPU on the system.
In kernels built with CONFIG_RCU_NOCB_CPU=y, set
the specified list of CPUs to be no-callback CPUs.

5
kernel/rcu/tree_plugin.h
View File

@ -1776,7 +1776,10 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp)
static int __init rcu_nocb_setup(char *str)
{
alloc_bootmem_cpumask_var(&rcu_nocb_mask);
cpulist_parse(str, rcu_nocb_mask);
if (!strcasecmp(str, "all"))
cpumask_setall(rcu_nocb_mask);
else
cpulist_parse(str, rcu_nocb_mask);
return 1;
}
__setup("rcu_nocbs=", rcu_nocb_setup);