Re: [PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose

From: Uladzislau Rezki <urezki@gmail.com>
To: Roman Gushchin <guro@fb.com>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Hillf Danton <hdanton@sina.com>, Michal Hocko <mhocko@suse.com>,
	Matthew Wilcox <willy@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Garnier <thgarnie@google.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joel Fernandes <joelaf@google.com>,
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose
Date: Mon, 3 Jun 2019 19:53:12 +0200	[thread overview]
Message-ID: <20190603175312.72td46uahgchfgma@pc636> (raw)
In-Reply-To: <20190529163435.GC3228@tower.DHCP.thefacebook.com>

Hello, Roman!

On Wed, May 29, 2019 at 04:34:40PM +0000, Roman Gushchin wrote:
> On Wed, May 29, 2019 at 04:27:15PM +0200, Uladzislau Rezki wrote:
> > Hello, Roman!
> > 
> > > On Mon, May 27, 2019 at 11:38:40AM +0200, Uladzislau Rezki (Sony) wrote:
> > > > Refactor the NE_FIT_TYPE split case when it comes to an
> > > > allocation of one extra object. We need it in order to
> > > > build a remaining space.
> > > > 
> > > > Introduce ne_fit_preload()/ne_fit_preload_end() functions
> > > > for preloading one extra vmap_area object to ensure that
> > > > we have it available when fit type is NE_FIT_TYPE.
> > > > 
> > > > The preload is done per CPU in non-atomic context thus with
> > > > GFP_KERNEL allocation masks. More permissive parameters can
> > > > be beneficial for systems which are suffer from high memory
> > > > pressure or low memory condition.
> > > > 
> > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > ---
> > > >  mm/vmalloc.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
> > > >  1 file changed, 76 insertions(+), 3 deletions(-)
> > > 
> > > Hi Uladzislau!
> > > 
> > > This patch generally looks good to me (see some nits below),
> > > but it would be really great to add some motivation, e.g. numbers.
> > > 
> > The main goal of this patch to get rid of using GFP_NOWAIT since it is
> > more restricted due to allocation from atomic context. IMHO, if we can
> > avoid of using it that is a right way to go.
> > 
> > From the other hand, as i mentioned before i have not seen any issues
> > with that on all my test systems during big rework. But it could be
> > beneficial for tiny systems where we do not have any swap and are
> > limited in memory size.
> 
> Ok, that makes sense to me. Is it possible to emulate such a tiny system
> on kvm and measure the benefits? Again, not a strong opinion here,
> but it will be easier to justify adding a good chunk of code.
> 
It seems it is not so straightforward as it looks like. I tried it before,
but usually the systems gets panic due to out of memory or just invokes
the OOM killer.

I will upload a new version of it, where i embed "preloading" logic directly
into alloc_vmap_area() function.

Thanks.

--
Vlad Rezki