linux-kernel - Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in kfree

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Date:   Fri, 17 Jan 2020 13:57:32 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: support kfree_bulk() interface in
 kfree_rcu()

On Fri, Jan 17, 2020 at 06:52:17PM +0100, Uladzislau Rezki wrote:
> > > > > But rcuperf uses a single block size, which turns into kfree_bulk() using
> > > > > a single slab, which results in good locality of reference.  So I have to
> > > > 
> > > > You meant a "single cache" category when you say "single slab"? Just to
> > > > mention, the number of slabs (in a single cache) when a large number of
> > > > objects are allocated is more than 1 (not single). With current rcuperf, I
> > > > see 100s of slabs (each slab being one page) in the kmalloc-32 cache. Each
> > > > slab contains around 128 objects of type kfree_rcu (24 byte object aligned to
> > > > 32-byte slab object).
> > > > 
> > > I think that is about using different slab caches to break locality. It
> > > makes sense, IMHO, because usually the system make use of different slabs,
> > > because of different object sizes. From the other hand i guess there are
> > > test cases when only one slab gets used.
> > 
> > I was wondering about "locality". A cache can be split into many slabs. Only
> > the data on a page is local (contiguous). If there are a large number of
> > objects, then it goes to a new slab (on the same cache). At least on the
> > kmalloc slabs, there is only 1 slab per page. So for example, if on
> > kmalloc-32 slab, there are more than 128 objects, then it goes to a different
> > slab / page. So how is there still locality?
> > 
> Hmm.. On a high level:
> 
> one slab cache manages a specific object size, i.e. the slab memory consists of
> contiguous pages(when increased probably not) of memory(4096 bytes or so) divided
> into equal object size. For example when kmalloc() gets called, the appropriate
> cache size(slab that serves only specific size) is selected and an object assigned
> from it is returned.
> 
> But that is theory and i have not deeply analyzed how the SLAB works internally,
> so i can be wrong :)
> 
> You mentioned 128 objects per one slab in the kmalloc-32 slab-cache. But all of
> them follows each other, i mean it is sequential and is like regular array. In

Yes, for these 128 objects it is sequential. But the next 128 could be on
some other page is what I was saying  And we are allocating 10s of 1000s of
objects in this test.  (I believe pages are sequential only per slab and not
for a different slab within same cache).

> that sense freeing can be beneficial because when an access is done to any object
> whole CPU cache-line is fetched(if it was not before), usually it is 64K.

You mean size of the whole L1 cache right? cachelines are in the order of bytes.

> That is what i meant "locality". In order to "break it" i meant to allocate from
> different slabs to see how kfree_slub() behaves in that sense, what is more real
> scenario and workload, i think.

Ok, agreed.
(BTW I do agree your patch is beneficial, just wanted to get the slab
discussion right).

thanks,

 - Joel