Re: [RFC 07/13] sched: Reduce stack size requirements in kernel/sched.c

[Mike Travis]

Peter Zijlstra wrote:
> On Sat, 2008-09-06 at 16:50 -0700, Mike Travis wrote:
>> plain text document attachment (stack-hogs-kernel_sched_c)
>> * Make the following changes to kernel/sched.c functions:
>>
>>     - use node_to_cpumask_ptr in place of node_to_cpumask
>>     - use get_cpumask_var for temporary cpumask_t variables
>>     - use alloc_cpumask_ptr where available
>>
>>   * Remove special code for SCHED_CPUMASK_ALLOC and use CPUMASK_ALLOC
>>     from linux/cpumask.h.
>>
>>   * The resultant stack savings are:
>>
>>     ====== Stack (-l 100)
>>
>> 	1 - initial
>> 	2 - stack-hogs-kernel_sched_c
>> 	'.' is less than the limit(100)
>>
>>        .1.    .2.    ..final..
>>       2216  -1536 680   -69%  __build_sched_domains
>>       1592  -1592   .  -100%  move_task_off_dead_cpu
>>       1096  -1096   .  -100%  sched_balance_self
>>       1032  -1032   .  -100%  sched_setaffinity
>>        616   -616   .  -100%  rebalance_domains
>>        552   -552   .  -100%  free_sched_groups
>>        512   -512   .  -100%  cpu_to_allnodes_group
>>       7616  -6936 680   -91%  Totals
>>
>>
>> Applies to linux-2.6.tip/master.
>>
>> Signed-off-by: Mike Travis <travis@xxxxxxx>
>> ---
>>  kernel/sched.c |  151 ++++++++++++++++++++++++++++++---------------------------
>>  1 file changed, 81 insertions(+), 70 deletions(-)
>>
>> --- linux-2.6.tip.orig/kernel/sched.c
>> +++ linux-2.6.tip/kernel/sched.c
>> @@ -70,6 +70,7 @@
>>  #include <linux/bootmem.h>
>>  #include <linux/debugfs.h>
>>  #include <linux/ctype.h>
>> +#include <linux/cpumask_ptr.h>
>>  #include <linux/ftrace.h>
>>  #include <trace/sched.h>
>>  
>> @@ -117,6 +118,12 @@
>>   */
>>  #define RUNTIME_INF	((u64)~0ULL)
>>  
>> +/*
>> + * temp cpumask variables
>> + */
>> +static DEFINE_PER_CPUMASK(temp_cpumask_1);
>> +static DEFINE_PER_CPUMASK(temp_cpumask_2);
> 
> Yuck, that relies on turning preemption off everywhere you want to use
> those.
> 
> 
>> @@ -5384,11 +5400,14 @@ out_unlock:
>>  
>>  long sched_setaffinity(pid_t pid, const cpumask_t *in_mask)
>>  {
>> -	cpumask_t cpus_allowed;
>> -	cpumask_t new_mask = *in_mask;
>> +	cpumask_ptr cpus_allowed;
>> +	cpumask_ptr new_mask;
>>  	struct task_struct *p;
>>  	int retval;
>>  
>> +	get_cpumask_var(cpus_allowed, temp_cpumask_1);
>> +	get_cpumask_var(new_mask, temp_cpumask_2);
>> +	*new_mask = *in_mask;
>>  	get_online_cpus();
>>  	read_lock(&tasklist_lock);
> 
> BUG!
> 
> get_online_cpus() can sleep, but you just disabled preemption with those
> get_cpumask_var() horribles!
> 
> Couldn't be arsed to look through the rest, but I really hate this
> cpumask_ptr() stuff that relies on disabling preemption.
> 
> NAK

Yeah, I really agree as well.  But I wanted to start playing with using
cpumask_t pointers in some fairly straight forward manner.  Linus's and
Ingo's suggestion to just bite the bullet and redefine the cpumask_t 
would force a lot of changes to be made, but perhaps that's really the
way to go.

As to obtaining temp cpumask_t's (both early and late), perhaps a pool of
them would be better?  I believe it could be done similar to alloc_bootmem
(but much simpler), and I don't think there's enough nesting to require a
very large pool.  (4 was the largest depth I could find in io_apic.c.)  Of
course, with preemption enabled then other problems arise...

One other really big use was for the "allbutself" cpumask in the send_IPI
functions.  I think here, preemption is ok because the ownership of the
cpumask temp is very short lived.

But thanks for pointing out the get_online_cpus problem.  I did try and
chase down as many call trees as I could, but I obviously missed one
important one.

And thanks for looking it over!
Mike

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