linux-kernel - Re: [PATCH tip/core/rcu 14/15] rcu/tree: Allocate a page when caller is preemptible

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Message-ID: <20200930123153.GA18005@pc636>
Date:   Wed, 30 Sep 2020 14:31:53 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        mgorman@...hsingularity.net, torvalds@...ux-foundation.org,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH tip/core/rcu 14/15] rcu/tree: Allocate a page when caller
 is preemptible

>
> > > What is the point of calling kmalloc  for a PAGE_SIZE object? Wouldn't
> > > using the page allocator directly be better?
> > 
> > Well, you guys gave me considerable heat about abusing internal allocator
> > interfaces, and kmalloc() and kfree() seem to be about as non-internal
> > as you can get and still be invoking the allocator.  ;-)
> 
> alloc_pages resp. __get_free_pages is a normal page allocator interface
> to use for page size granular allocations. kmalloc is for more fine
> grained allocations.
>
I do not have a strong opinion here but i would tend to using of the page
allocator directly. Because we operate on a page level where the page
allocator is supposed to be used. From the other hand i saw a slightly
better performance in case of SLAB only. I think that is because of
a) slab caches; b) more objects which can be cached. But i have one more
concern about using of kmalloc(). That is about fragmentations it can
cause with PAGE_SIZE requests.

As for SLUB it was identical whereas the SLOB i din not check.

--
Vlad Rezki