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Wed, 06 Mar 2019 08:25:21 -0800 (PST) Received: from localhost ([2620:0:1004:1100:cca9:fccc:8667:9bdc]) by smtp.gmail.com with ESMTPSA id w8sm1626418qkw.80.2019.03.06.08.25.20 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 06 Mar 2019 08:25:20 -0800 (PST) Date: Wed, 6 Mar 2019 11:25:19 -0500 From: Joel Fernandes To: Uladzislau Rezki Cc: Andrew Morton , Michal Hocko , Matthew Wilcox , linux-mm@kvack.org, LKML , Thomas Garnier , Oleksiy Avramchenko , Steven Rostedt , Thomas Gleixner , Ingo Molnar , Tejun Heo Subject: Re: [PATCH v1 2/2] mm: add priority threshold to __purge_vmap_area_lazy() Message-ID: <20190306162519.GB193418@google.com> References: <20190124115648.9433-1-urezki@gmail.com> <20190124115648.9433-3-urezki@gmail.com> <20190128224528.GB38107@google.com> <20190129173936.4sscooiybzbhos77@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190129173936.4sscooiybzbhos77@pc636> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 29, 2019 at 06:39:36PM +0100, Uladzislau Rezki wrote: > On Mon, Jan 28, 2019 at 05:45:28PM -0500, Joel Fernandes wrote: > > On Thu, Jan 24, 2019 at 12:56:48PM +0100, Uladzislau Rezki (Sony) wrote: > > > commit 763b218ddfaf ("mm: add preempt points into > > > __purge_vmap_area_lazy()") > > > > > > introduced some preempt points, one of those is making an > > > allocation more prioritized over lazy free of vmap areas. > > > > > > Prioritizing an allocation over freeing does not work well > > > all the time, i.e. it should be rather a compromise. > > > > > > 1) Number of lazy pages directly influence on busy list length > > > thus on operations like: allocation, lookup, unmap, remove, etc. > > > > > > 2) Under heavy stress of vmalloc subsystem i run into a situation > > > when memory usage gets increased hitting out_of_memory -> panic > > > state due to completely blocking of logic that frees vmap areas > > > in the __purge_vmap_area_lazy() function. > > > > > > Establish a threshold passing which the freeing is prioritized > > > back over allocation creating a balance between each other. > > > > I'm a bit concerned that this will introduce the latency back if vmap_lazy_nr > > is greater than half of lazy_max_pages(). Which IIUC will be more likely if > > the number of CPUs is large. > > > The threshold that we establish is two times more than lazy_max_pages(), > i.e. in case of 4 system CPUs lazy_max_pages() is 24576, therefore the > threshold is 49152, if PAGE_SIZE is 4096. > > It means that we allow rescheduling if vmap_lazy_nr < 49152. If vmap_lazy_nr > is higher then we forbid rescheduling and free areas until it becomes lower > again to stabilize the system. By doing that, we will not allow vmap_lazy_nr > to be enormously increased. Sorry for late reply. This sounds reasonable. Such an extreme situation of vmap_lazy_nr being twice the lazy_max_pages() is probably only possible using a stress test anyway since (hopefully) the try_purge_vmap_area_lazy() call is happening often enough to keep the vmap_lazy_nr low. Have you experimented with what is the highest threshold that prevents the issues you're seeing? Have you tried 3x or 4x the vmap_lazy_nr? I also wonder what is the cost these days of the global TLB flush on the most common Linux architectures and if the whole purge vmap_area lazy stuff is starting to get a bit dated, and if we can do the purging inline as areas are freed. There is a cost to having this mechanism too as you said, which is as the list size grows, all other operations also take time. thanks, - Joel > > In fact, when vmap_lazy_nr is high, that's when the latency will be the worst > > so one could say that that's when you *should* reschedule since the frees are > > taking too long and hurting real-time tasks. > > > > Could this be better solved by tweaking lazy_max_pages() such that purging is > > more aggressive? > > > > Another approach could be to detect the scenario you brought up (allocations > > happening faster than free), somehow, and avoid a reschedule? > > > This is what i am trying to achieve by this change. > > Thank you for your comments. > > -- > Vlad Rezki > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > --- > > > mm/vmalloc.c | 18 ++++++++++++------ > > > 1 file changed, 12 insertions(+), 6 deletions(-) > > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > > > index fb4fb5fcee74..abe83f885069 100644 > > > --- a/mm/vmalloc.c > > > +++ b/mm/vmalloc.c > > > @@ -661,23 +661,27 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end) > > > struct llist_node *valist; > > > struct vmap_area *va; > > > struct vmap_area *n_va; > > > - bool do_free = false; > > > + int resched_threshold; > > > > > > lockdep_assert_held(&vmap_purge_lock); > > > > > > valist = llist_del_all(&vmap_purge_list); > > > + if (unlikely(valist == NULL)) > > > + return false; > > > + > > > + /* > > > + * TODO: to calculate a flush range without looping. > > > + * The list can be up to lazy_max_pages() elements. > > > + */ > > > llist_for_each_entry(va, valist, purge_list) { > > > if (va->va_start < start) > > > start = va->va_start; > > > if (va->va_end > end) > > > end = va->va_end; > > > - do_free = true; > > > } > > > > > > - if (!do_free) > > > - return false; > > > - > > > flush_tlb_kernel_range(start, end); > > > + resched_threshold = (int) lazy_max_pages() << 1; > > > > > > spin_lock(&vmap_area_lock); > > > llist_for_each_entry_safe(va, n_va, valist, purge_list) { > > > @@ -685,7 +689,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end) > > > > > > __free_vmap_area(va); > > > atomic_sub(nr, &vmap_lazy_nr); > > > - cond_resched_lock(&vmap_area_lock); > > > + > > > + if (atomic_read(&vmap_lazy_nr) < resched_threshold) > > > + cond_resched_lock(&vmap_area_lock); > > > } > > > spin_unlock(&vmap_area_lock); > > > return true; > > > -- > > > 2.11.0 > > >