From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 22 May 2019 14:07:05 +0200 (CEST) From: Sebastian Ott Subject: Re: [PATCH 05/10] s390/cio: introduce DMA pools to cio In-Reply-To: <20190520141312.4e3a2d36.pasic@linux.ibm.com> References: <20190426183245.37939-1-pasic@linux.ibm.com> <20190426183245.37939-6-pasic@linux.ibm.com> <20190508232210.5a555caa.pasic@linux.ibm.com> <20190509121106.48aa04db.cohuck@redhat.com> <20190510001112.479b2fd7.pasic@linux.ibm.com> <20190510161013.7e697337.cohuck@redhat.com> <20190512202256.5517592d.pasic@linux.ibm.com> <20190520141312.4e3a2d36.pasic@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Message-Id: Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: Halil Pasic Cc: Cornelia Huck , kvm@vger.kernel.org, linux-s390@vger.kernel.org, Martin Schwidefsky , virtualization@lists.linux-foundation.org, "Michael S. Tsirkin" , Christoph Hellwig , Thomas Huth , Christian Borntraeger , Viktor Mihajlovski , Vasily Gorbik , Janosch Frank , Claudio Imbrenda , Farhan Ali , Eric Farman , Michael Mueller List-ID: On Mon, 20 May 2019, Halil Pasic wrote: > On Thu, 16 May 2019 15:59:22 +0200 (CEST) > Sebastian Ott wrote: > > We only have a couple of users for airq_iv: > > > > virtio_ccw.c: 2K bits > > You mean a single allocation is 2k bits (VIRTIO_IV_BITS = 256 * 8)? My > understanding is that the upper bound is more like: > MAX_AIRQ_AREAS * VIRTIO_IV_BITS = 20 * 256 * 8 = 40960 bits. > > In practice it is most likely just 2k. > > > > > pci with floating IRQs: <= 2K (for the per-function bit vectors) > > 1..4K (for the summary bit vector) > > > > As far as I can tell with virtio_pci arch_setup_msi_irqs() gets called > once per device and allocates a small number of bits (2 and 3 in my > test, it may depend on #virtqueues, but I did not check). > > So for an upper bound we would have to multiply with the upper bound of > pci devices/functions. What is the upper bound on the number of > functions? > > > pci with CPU directed IRQs: 2K (for the per-CPU bit vectors) > > 1..nr_cpu (for the summary bit vector) > > > > I guess this is the same. > > > > > The options are: > > * page allocations for everything > > Worst case we need 20 + #max_pci_dev pages. At the moment we allocate > from ZONE_DMA (!) and waste a lot. > > > * dma_pool for AIRQ_IV_CACHELINE ,gen_pool for others > > I prefer this. Explanation follows. > > > * dma_pool for everything > > > > Less waste by factor factor 16. > > > I think we should do option 3 and use a dma_pool with cachesize > > alignment for everything (as a prerequisite we have to limit > > config PCI_NR_FUNCTIONS to 2K - but that is not a real constraint). > > I prefer option 3 because it is conceptually the smallest change, and ^ 2 > provides the behavior which is closest to the current one. I can see that this is the smallest change on top of the current implementation. I'm good with doing that and looking for further simplification/unification later. > Commit 414cbd1e3d14 "s390/airq: provide cacheline aligned > ivs" (Sebastian Ott, 2019-02-27) could have been smaller had you implemented > 'kmem_cache for everything' (and I would have had just to replace kmem_cache with > dma_cache to achieve option 3). For some reason you decided to keep the > iv->vector = kzalloc(size, GFP_KERNEL) code-path and make the client code request > iv->vector = kmem_cache_zalloc(airq_iv_cache, GFP_KERNEL) explicitly, using a flag > which you only decided to use for directed pci irqs AFAICT. > > My understanding of these decisions, and especially of the rationale > behind commit 414cbd1e3d14 is limited. I introduced per cpu interrupt vectors and wanted to prevent 2 CPUs from sharing data from the same cacheline. No other user of the airq stuff had this need. If I had been aware of the additional complexity we would add on top of that maybe I would have made a different decision.