From a48c1b89c849a390be9e5b210759369b35777b39 Mon Sep 17 00:00:00 2001
From: Daniel Black <daniel@mariadb.org>
Date: Thu, 23 Nov 2023 13:03:01 +1100
Subject: [PATCH] MDEV-24670 memory pressure - eventfd rather than pipe

Eventfds have a simplier interface and are one file
descriptor rather than two.

Reuse the patten of the accepting socket connections
by testing for abort after a poll returns. This way
the same event descriptor can be used for Quit
and debugging trigger.

Also correct the registration of mem pressure file
descriptors.
---
 storage/innobase/buf/buf0buf.cc | 59 +++++++++++++++------------------
 1 file changed, 27 insertions(+), 32 deletions(-)

diff --git a/storage/innobase/buf/buf0buf.cc b/storage/innobase/buf/buf0buf.cc
index 3f1df10ac00..7e584ae07c3 100644
--- a/storage/innobase/buf/buf0buf.cc
+++ b/storage/innobase/buf/buf0buf.cc
@@ -744,6 +744,7 @@ bool buf_page_is_corrupted(bool check_lsn, const byte *read_buf,
 
 #ifdef __linux__
 #include <poll.h>
+#include <sys/eventfd.h>
 #include <fstream>
 
 /** Memory Pressure
@@ -752,14 +753,16 @@ based off https://www.kernel.org/doc/html/latest/accounting/psi.html#pressure-in
 and https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#memory */
 class mem_pressure
 {
-  /* triggers + pipe */
+  /* triggers + eventfd */
   struct pollfd m_fds[3];
   nfds_t m_num_fds;
-  int m_pipe[2]= {-1, -1};
+  int m_event_fd= -1;
+  Atomic_relaxed<bool> m_abort= false;
 
   std::thread m_thd;
-  /* mem pressure garbarge collection resticted to interval */
+  /* mem pressure garbage collection restricted to interval */
   static constexpr ulonglong max_interval_us= 60*1000000;
+
 public:
   mem_pressure() : m_num_fds(0) {}
 
@@ -796,7 +799,7 @@ public:
         shutdown();
         return false;
       }
-      my_register_filename(m_pipe[0], memcgroup.c_str(), FILE_BY_OPEN, 0, MYF(0));
+      my_register_filename(m_fds[m_num_fds].fd, memcgroup.c_str(), FILE_BY_OPEN, 0, MYF(0));
       ssize_t slen= strlen(*trig);
       if (write(m_fds[m_num_fds].fd, *trig, slen) < slen)
       {
@@ -810,15 +813,14 @@ public:
     if (m_num_fds < 1)
       return false;
 
-    if (pipe2(m_pipe, O_CLOEXEC | O_DIRECT))
+    if ((m_event_fd= eventfd(0, EFD_CLOEXEC|EFD_NONBLOCK)) == -1)
     {
-      sql_print_warning("InnoDB: No memory pressure - can't create pipe");
+      sql_print_warning("InnoDB: No memory pressure - can't create eventfd");
       shutdown();
       return false;
     }
-    my_register_filename(m_pipe[0], "mem_pressure_read", FILE_BY_DUP, 0, MYF(0));
-    my_register_filename(m_pipe[1], "mem_pressure_write", FILE_BY_DUP, 0, MYF(0));
-    m_fds[m_num_fds].fd= m_pipe[0];
+    my_register_filename(m_event_fd, "mem_pressure_eventfd", FILE_BY_DUP, 0, MYF(0));
+    m_fds[m_num_fds].fd= m_event_fd;
     m_fds[m_num_fds].events= POLLIN;
     m_num_fds++;
     m_thd= std::thread(pressure_routine, this);
@@ -827,18 +829,13 @@ public:
 
   void shutdown()
   {
+    /* m_event_fd is in this list */
     while (m_num_fds)
     {
       m_num_fds--;
       my_close(m_fds[m_num_fds].fd, MYF(MY_WME));
       m_fds[m_num_fds].fd= -1;
     }
-    /* note m_pipe[0] closed in above loop */
-    if (m_pipe[1] >= 0)
-    {
-      my_close(m_pipe[1], MYF(MY_WME));
-      m_pipe[1]= -1;
-    }
   }
 
   static void pressure_routine(mem_pressure *m);
@@ -846,8 +843,8 @@ public:
 #ifdef UNIV_DEBUG
   void trigger_collection()
   {
-    const char c= 'G';
-    if (m_pipe[1] >=0 && write(m_pipe[1], &c, 1) < 1)
+    uint64_t u= 1;
+    if (m_event_fd >=0 && write(m_event_fd, &u, sizeof(uint64_t)) != sizeof(uint64_t))
       sql_print_information("InnoDB: (Debug) Failed to trigger memory pressure");
     else /* assumed failed to meet intialization criteria, so trigger directy */
       buf_pool.garbage_collect();
@@ -856,8 +853,9 @@ public:
 
   void quit()
   {
-    const char c= 'Q';
-    if (m_num_fds && write(m_pipe[1], &c, 1) < 1)
+    uint64_t u= 1;
+    m_abort= true;
+    if (write(m_event_fd, &u, sizeof(uint64_t)) != sizeof(uint64_t))
       sql_print_warning("InnoDB: Failed to write memory pressure quit message");
   }
 
@@ -896,7 +894,7 @@ void mem_pressure::pressure_routine(mem_pressure *m)
   }
 
   ulonglong last= microsecond_interval_timer() - max_interval_us;
-  while (1)
+  while (!m->m_abort)
   {
     if (poll(&m->m_fds[0], m->m_num_fds, -1) < 0)
     {
@@ -909,6 +907,9 @@ void mem_pressure::pressure_routine(mem_pressure *m)
         break;
       }
     }
+    if (!m->m_abort)
+      break;
+
     for (pollfd &p : st_::span<pollfd>(m->m_fds, m->m_num_fds))
     {
       if (p.revents & POLLPRI)
@@ -921,23 +922,17 @@ void mem_pressure::pressure_routine(mem_pressure *m)
         }
       }
 
+#ifdef UNIV_DEBUG
       if (p.revents & POLLIN)
       {
-        char c= '\0';
-        /* signal to quit */
-        if (read(p.fd, &c, 1) >=0)
-          switch (c) {
-          case 'Q':
-            goto shutdown;
-#ifdef UNIV_DEBUG
-          case 'G':
-            buf_pool.garbage_collect();
-#endif
-          }
+        uint64_t u;
+        /* we haven't aborted, so this must be a debug trigger */
+        if (read(p.fd, &u, sizeof(u)) >=0)
+          buf_pool.garbage_collect();
       }
+#endif
     }
   }
-shutdown:
   m->shutdown();
 
   my_thread_end();