Re: [RFC 0/3] Slab Defrag / Slab Targeted Reclaim and general Slab API changes

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Eric Dumazet a écrit :
Christoph Lameter a écrit :


G. Being able to track the number of pages in a kmem_cache


If you look at fs/buffer.c, you'll notice the bh_accounting, recalc_bh_state() that might be overkill for large SMP configurations, when the real concern is
to be able to limit the bh's not to exceed 10% of LOWMEM.

Adding a callback in slab_ops to track total number of pages in use by a given
kmem_cache would be good.

Such functionality exists internal to SLUB and in the reporting tool. I can export that function if you need it.

Same thing for fs/file_table.c : nr_file logic
(percpu_counter_dec()/percpu_counter_inc() for each file open/close) could be simplified if we could just count the pages in use by filp_cachep kmem_cache.
The get_nr_files() thing is not worth the pain.

Sure. What exactly do you want? The absolute number of pages of memory that the slab is using?

    kmem_cache_pages_in_use(struct kmem_cache *) ?

The call will not be too lightweight since we will have to loop over all nodes and add the counters in each per node struct for allocates slabs.



On a typical system, number of pages for 'filp' kmem_cache tends to be stable

-bash-2.05b# grep filp /proc/slabinfo
filp 234727 374100 256 15 1 : tunables 120 60 8 : slabdata 24940 24940 135
-bash-2.05b# grep filp /proc/slabinfo
filp 234776 374100 256 15 1 : tunables 120 60 8 : slabdata 24940 24940 168
-bash-2.05b# grep filp /proc/slabinfo
filp 234728 374100 256 15 1 : tunables 120 60 8 : slabdata 24940 24940 180
-bash-2.05b# grep filp /proc/slabinfo
filp 234724 374100 256 15 1 : tunables 120 60 8 : slabdata 24940 24940 174

So revert nr_files logic to a single integer would be enough, even for NUMA

int nr_pages_used_by_filp;
int nr_pages_filp_limit;
int filp_in_danger __read_mostly;

static void callback_pages_in_use_by_filp(int inc)
{
    int in_danger;

    nr_pages_used_by_filp += inc;

    in_danger = nr_pages_used_by_filp >= nr_pages_filp_limit;
    if (in_danger != filp_in_danger)
        filp_in_danger = in_danger;
}

struct file *get_empty_filp(void)
{
...
if (filp_in_danger && !capable(CAP_SYS_ADMIN))
    goto over;

...
}


void __init files_init(unsigned long mempages)
{
...
nr_pages_filp_limit = (mempages * 10) / 100; /* 10% for filp use */
...
}


This wont work of course : Once the pages limit is hit, file allocations would be forbidden until cache is shrinked.

Maybe callback should return a status, so that SLAB / SLUB can return ENOMEM

static int callback_pages_in_use_by_filp(int inc)
{
    int in_danger;

    nr_pages_used_by_filp += inc;

    in_danger = nr_pages_used_by_filp >= nr_pages_filp_limit;
    if (unlikely(in_danger != filp_in_danger))
        filp_in_danger = in_danger;
    if (unlikely(in_danger && inc > 0 && !capable(CAP_SYS_ADMIN))) {
        nr_pages_used_by_filp -= inc;
        return -1;
    }
    return 0;
}

No more tests in get_empty_filp() : just call kmem_cache_alloc()

struct file *get_empty_filp(void)
{
...
f = kmem_cache_alloc(filp_cachep, GFP_KERNEL);
...
}

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