[Xen-changelog] [xen staging] libxl: event: poller pipe optimisation

[patchbot@xxxxxxx]

commit 67a97ca9550f117bd4e690a01c34398ddc1bf0d6
Author:     Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
AuthorDate: Fri Jan 10 13:11:46 2020 +0000
Commit:     Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
CommitDate: Mon Jan 27 16:03:17 2020 +0000

    libxl: event: poller pipe optimisation
    
    Track in userland whether the poller pipe is nonempty.  This saves us
    writing many many bytes to the pipe if nothing ever reads them.
    
    This is going to be relevant in a moment, where we are going to create
    a situation where this will happen quite a lot.
    
    Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    Tested-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 tools/libxl/libxl_event.c    | 3 +++
 tools/libxl/libxl_internal.h | 1 +
 2 files changed, 4 insertions(+)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index b50d4e5074..3e76fa5af5 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -1417,6 +1417,7 @@ static void afterpoll_internal(libxl__egc *egc, 
libxl__poller *poller,
     }
 
     if (afterpoll_check_fd(poller,fds,nfds, poller->wakeup_pipe[0],POLLIN)) {
+        poller->pipe_nonempty = 0;
         int e = libxl__self_pipe_eatall(poller->wakeup_pipe[0]);
         if (e) LIBXL__EVENT_DISASTER(gc, "read wakeup", e, 0);
     }
@@ -1809,6 +1810,8 @@ void libxl__poller_put(libxl_ctx *ctx, libxl__poller *p)
 
 void libxl__poller_wakeup(libxl__gc *gc, libxl__poller *p)
 {
+    if (p->pipe_nonempty) return;
+    p->pipe_nonempty = 1;
     int e = libxl__self_pipe_wakeup(p->wakeup_pipe[1]);
     if (e) LIBXL__EVENT_DISASTER(gc, "cannot poke watch pipe", e, 0);
 }
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index f2ff5e6c2d..2555aa4575 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -625,6 +625,7 @@ struct libxl__poller {
     int (*fd_rindices)[3]; /* see libxl_event.c:beforepoll_internal */
 
     int wakeup_pipe[2]; /* 0 means no fd allocated */
+    bool pipe_nonempty;
 
     /*
      * We also use the poller to record whether any fds have been
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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