Messages in this thread | | | Date | Wed, 29 Mar 2023 08:47:35 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions |
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On Wed, 29 Mar 2023 13:23:01 +0100 Vincent Donnefort <vdonnefort@google.com> wrote:
> On Wed, Mar 29, 2023 at 07:03:53AM -0400, Steven Rostedt wrote: > > On Wed, 29 Mar 2023 10:19:44 +0100 > > Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > I've been playing with this a bit, and I'm thinking, do we need the > > > > data_pages[] array on the meta page? > > > > > > > > I noticed that I'm not even using it. > > > > > > > > Currently, we need to do a ioctl every time we finish with the reader page, > > > > and that updates the reader_page in the meta data to point to the next page > > > > to read. When do we need to look at the data_start section? > > > > > > This is for non-consuming read, to get all the pages in order. > > > > Yeah, I was trying to see how a non consuming read would work, and was > > having issues figuring that out without the tail page being updated. > > Would the userspace really need to know where is the tail page? It can just stop > whenever it finds out a page doesn't have any events, and make sure it does not > loop once back to the head?
I'm trying to come up with a possible algorithm that doesn't need ioctls. It would need to know if the writer moved or not. Probably need a counter that gets incremented every time the writer goes to a new page.
Having the tail page was just a convenient way to know where the end is.
> > > > > > > > > If we remove this section we would lose this ability ... but we'd also simplify > > > the code by a good order of magnitude (don't need the update ioctl anymore, no > > > need to keep those pages in order and everything can fit a 0-order meta-page). > > > And the non-consuming read doesn't bring much to the user over the pipe version. > > > > > > This will although impact our hypervisor tracing which will only be able to > > > expose trace_pipe interfaces. But I don't think it is a problem, all userspace > > > tools only relying on consuming read anyway. > > > > > > So if you're happy dropping this support, let's get rid of it. > > > > I don't really want to get rid of it, but perhaps break it up where we > > don't have it in the first release, but add it in a second one. That will > > also make sure that we can expand the API if necessary (one reason I wanted > > the "data_start" in the first place). > > > > Let's drop it for now, but be able to add it later, an have the current > > structure be: > > Ok, I will prepare a V3 accordingly. > > > > > struct ring_buffer_meta_page_header { > > #if __BITS_PER_LONG == 64 > > __u64 entries; > > __u64 overrun; > > #else > > __u32 entries; > > __u32 overrun; > > #endif > > __u32 pages_touched; > > __u32 meta_page_size; > > __u32 reader_page; /* page ID for the reader page */ > > __u32 nr_data_pages; /* doesn't take into account the reader_page */ > > }; > > > > BTW, shouldn't the nr_data_pages take into account the reader page? As it > > is part of the array we traverse isn't it? > > It depends if the reader page has ever been swapped out. If yes, the reader > would have to start from reader_page and then switch to the data_pages. > Which sounds like a fiddly interface for the userspace. > > So yeah, consuming-read only feels like a better start. >
I agree. I'd like to get something in that can be extended, but simple enough that it's not too much of a barrier wrt getting the API correct.
-- Steve
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