On Mon, Jun 8, 2015 at 8:48 AM, Vinod Koul <vinod.koul@xxxxxxxxx> wrote: > On Mon, Jun 08, 2015 at 07:44:43PM +0800, Jiang Liu wrote: >> On 2015/6/8 18:42, Vinod Koul wrote: >> > On Tue, Jun 02, 2015 at 02:37:31PM +0800, Jiang Liu wrote: >> >> Ccing Rafael, it's ACPI hotplug related. >> >> >> >> On 2015/6/2 14:36, Jiang Liu wrote: >> >>> The dmaengine core assumes that async DMA devices will only be removed >> >>> when they not used anymore, or it assumes dma_async_device_unregister() >> >>> will only be called by dma driver exit routines. But this assumption is >> >>> not true for the IOAT driver, which calls dma_async_device_unregister() >> >>> from ioat_remove(). So current IOAT driver doesn't support device >> >>> hot-removal because it may cause system crash to hot-remove an inuse >> >>> IOAT device. >> >>> >> >>> To support CPU socket hot-removal, all PCI devices, including IOAT >> >>> devices embedded in the socket, will be hot-removed. The idea solution >> >>> is to enhance the dmaengine core and IOAT driver to support hot-removal, >> >>> but that's too hard. >> >>> >> >>> This patch implements a hack to disable IOAT devices under hotplug-capable >> >>> CPU socket so it won't break socket hot-removal. >> >>> >> > So below looks okay though I wonder how hard would it be to fix hot unplug ? >> Hi Vinod, >> Thanks for review. About three years ago I worked out a >> patch set to enhance the dmaengine core and ioat device driver to >> support hot-removal. But it has been rejected due to concerns about >> performance penalty caused by usage tracking. >> To support hot-removal, we need to track dma channel usage >> and a way to reclaim dma channels when hot-removing. This may cause >> sensible performance penalty. Recently I have tried again but still >> haven't find a way to support hot-removal. So eventually I suggest >> to disable IOAT device on hot-plug capable systems. > > Or on a different mechanism, take the module reference on the channel > allocation and release it one channel release. > > That way we don't need to count and we ensure dmaengine module is removed > only when users have stopped using the device... This was one of the first "features" of dmaengine I deleted. There's no clean / reliable way to support general purpose dma-offload and time bounded hot-removal. Multiple clients may be using a channel in varied contexts so you both need to tell them to stop and wait for them to acknowledge. On platforms with socket hotplug I would expect the cpu to almost always be faster than an ioatdma offload. So, fwiw, I think hotplug capability is more useful to the platform than ioatdma offload. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
