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Tue, 29 Jan 2019 09:39:45 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 29 Jan 2019 18:39:36 +0100 To: Joel Fernandes Cc: "Uladzislau Rezki (Sony)" , 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: <20190129173936.4sscooiybzbhos77@pc636> References: <20190124115648.9433-1-urezki@gmail.com> <20190124115648.9433-3-urezki@gmail.com> <20190128224528.GB38107@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190128224528.GB38107@google.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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 > >