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

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Message-ID: <1446605804.17120.179.camel@tzanussi-mobl.amr.corp.intel.com>
Date:	Tue, 03 Nov 2015 20:56:44 -0600
From:	Tom Zanussi <tom.zanussi@...ux.intel.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	rostedt@...dmis.org, daniel.wagner@...-carit.de,
	masami.hiramatsu.pt@...achi.com, josh@...htriplett.org,
	andi@...stfloor.org, mathieu.desnoyers@...icios.com,
	peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 08/28] tracing: Add lock-free tracing_map

Hi Namhyung,

On Wed, 2015-11-04 at 11:26 +0900, Namhyung Kim wrote:
> 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(...);
> 

Yeah, I think that should work - let me try that in the next version...

Thanks,

Tom

> Thanks,
> Namhyung


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