Re: [PATCH v11 08/28] tracing: Add lock-free tracing_map

[Namhyung Kim]

Hi Tom,

On Tue, Nov 03, 2015 at 07:47:52PM -0600, Tom Zanussi wrote:
> Hi Namhyung,
> 
> On Mon, 2015-11-02 at 16:08 +0900, Namhyung Kim wrote:
> > I thought it'd be better if users can see which one is the real drop
> > or not.  IOW if drop count is much smaller than the normal event
> > count, [s]he might want to ignore the occasional drops.  Otherwise,
> > [s]he should restart with a bigger table.  This requires accurate
> > counts of events and drops though.
> > 
> 
> OK, how about the below - it basically moves the drops set/test/inc into
> tracing_map_insert(), as well as a total hits count.  So those values
> will be available for users to use in deciding whether to use the data
> or restart with a bigger table, and the loop is bailed out of only if no
> matching keys are found and there are drops, so callers can continue
> updating existing entries.

But if a key didn't get a desired index, it'd still fail to update..


> 
> Users who want the original behavior still get the NULL return and can
> stop calling tracing_map_insert() as before:
> 
> struct tracing_map_elt *tracing_map_insert(struct tracing_map *map, void *key)
> {
>         u32 idx, key_hash, test_key;
>         struct tracing_map_entry *entry;
> 
>         key_hash = jhash(key, map->key_size, 0);
>         if (key_hash == 0)
>                 key_hash = 1;
>         idx = key_hash >> (32 - (map->map_bits + 1));
> 
>         while (1) {
>                 idx &= (map->map_size - 1);
>                 entry = TRACING_MAP_ENTRY(map->map, idx);
>                 test_key = entry->key;
> 
>                 if (test_key && test_key == key_hash && entry->val &&
>                     keys_match(key, entry->val->key, map->key_size)) {
>                         atomic64_inc(&map->hits);
>                         return entry->val;
>                 }
> 
>                 if (atomic64_read(&map->drops)) {
>                         atomic64_inc(&map->drops);
>                         break;
>                 }

IMHO it should be removed.

> 
>                 if (!test_key && !cmpxchg(&entry->key, 0, key_hash)) {
>                         struct tracing_map_elt *elt;
> 
>                         elt = get_free_elt(map);
>                         if (!elt) {
>                                 atomic64_inc(&map->drops);

And reset entry->key here..

>                                 break;
>                         }
>                         memcpy(elt->key, key, map->key_size);
>                         entry->val = elt;
> 
>                         atomic64_inc(&map->hits);
>                         return entry->val;
>                 }
>                 idx++;
>         }
> 
>         return NULL;
> }


Then tracing_map_lookup() can be implemented like *_insert but bail out
from the loop if test_key is 0.  The caller might do like this:

	if (!atomic64_read(&map->drops))
		elt = tracing_map_insert(...);
	else
		elt = tracing_map_lookup(...);

Thanks,
Namhyung
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