[Devel] Re: [PATCH 1/4] Add notification about some major slab events

[Pavel Emelyanov]

[snip]

>> @@ -1555,6 +1650,11 @@ static void __always_inline *slab_alloc(
>>  	}
>>  	local_irq_restore(flags);
>>  
>> +	if (object && slub_alloc_notify(s, object, gfpflags) < 0) {
>> +		kmem_cache_free(s, object);
>> +		return NULL;
>> +	}
>> +
>>  	if (unlikely((gfpflags & __GFP_ZERO) && object))
>>  		memset(object, 0, c->objsize);
>>  
> 
> Please stay completely out of the fast path. No modifications to 
> slab_alloc or slab_free please. It is possible to force all allocations of 
> a particular slab of interest to use the slow path in __slab_alloc (maybe 
> as a result of the slab page allocation hook returning a certain result 
> code). See how the SLAB_DEBUG handling does it. You can adapt that and then do the 
> object checks in __slab_alloc.

I have run the kernbench test on the kernels with a) containers support
and b) containers and kmem accounting support but (!) turned off. The
results are:
                 a)             b)
Elapsed Time     768.500000     767.050000
User Time        679.360000     679.240000
System Time      87.020000      86.950000
Percent CPU      99.000000      99.000000
Context Switches 376891.000000  375407.000000
Sleeps           385377.000000  385426.000000

The test run was kernbench -n 1 -o 4 -M, the node is i386
DualCore Xeon 3.2GHz with 2Gb of RAM.

so the fast path is still fast, and we have two ways:
1. we keep the checks on the fastpath and have 0 overhead for
   unaccounted caches and some overhead for accounted;
2. we move the checks into the slow one and have 0 overhead for
   unaccounted caches and huge overhead for accounted.

I admit that I messed something, so shall I measure some 
other activity or use another HW?

Thanks,
Pavel