Requesting review about optimizing large guest start up time

["bauerchen(陈蒙蒙)" <bauerchen@tencent.com>]

From c882b155466313fcd85ac330a45a573e608b0d74 Mon Sep 17 00:00:00 2001
From: bauerchen <bauerchen@tencent.com>
Date: Tue, 11 Feb 2020 17:10:35 +0800
Subject: [PATCH] Optimize: large guest start-up in mem-prealloc
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[desc]:
    Large memory VM starts slowly when using -mem-prealloc, and
    there are some areas to optimize in current method;

    1、mmap will be used to alloc threads stack during create page
    clearing threads, and it will attempt mm->mmap_sem for write
    lock, but clearing threads have hold read lock, this competition
    will cause threads createion very slow;

    2、methods of calcuating pages for per threads is not well;if we use
    64 threads to split 160 hugepage,63 threads clear 2page,1 thread
    clear 34 page,so the entire speed is very slow;

    to solve the first problem,we add a mutex in thread function,and
    start all threads when all threads finished createion;
    and the second problem, we spread remainder to other threads,in
    situation that 160 hugepage and 64 threads, there are 32 threads
    clear 3 pages,and 32 threads clear 2 pages;
[test]:
    320G 84c VM start time can be reduced to 10s
    680G 84c VM start time can be reduced to 18s

Signed-off-by: bauerchen <bauerchen@tencent.com>
Reviewed-by:Pan Rui <ruippan@tencent.com>
Reviewed-by:Ivan Ren <ivanren@tencent.com>
---
 util/oslib-posix.c | 44 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 5a291cc..e97369b 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -76,6 +76,10 @@ static MemsetThread *memset_thread;
 static int memset_num_threads;
 static bool memset_thread_failed;
 
+static QemuMutex page_mutex;
+static QemuCond page_cond;
+static volatile bool thread_create_flag;
+
 int qemu_get_thread_id(void)
 {
 #if defined(__linux__)
@@ -403,6 +407,14 @@ static void *do_touch_pages(void *arg)
     MemsetThread *memset_args = (MemsetThread *)arg;
     sigset_t set, oldset;
 
+    /*wait for all threads create finished */
+    qemu_mutex_lock(&page_mutex);
+    while(!thread_create_flag){
+        qemu_cond_wait(&page_cond, &page_mutex);
+    }
+    qemu_mutex_unlock(&page_mutex);
+
+
     /* unblock SIGBUS */
     sigemptyset(&set);
     sigaddset(&set, SIGBUS);
@@ -448,30 +460,46 @@ static inline int get_memset_num_threads(int smp_cpus)
     return ret;
 }
 
+static void calc_page_per_thread(size_t numpages, int memset_threads, size_t *pages_per_thread){
+    int avg = numpages / memset_threads + 1;
+    int i = 0;
+    int last = avg * memset_threads - numpages;
+    for (i = 0; i < memset_threads; i++)
+    {
+        if(memset_threads - i <= last){
+            pages_per_thread[i] = avg - 1;
+        }else
+            pages_per_thread[i] = avg;
+    }
+}
+
 static bool touch_all_pages(char *area, size_t hpagesize, size_t numpages,
                             int smp_cpus)
 {
-    size_t numpages_per_thread;
-    size_t size_per_thread;
+    size_t *numpages_per_thread;
     char *addr = area;
     int i = 0;
 
     memset_thread_failed = false;
+    thread_create_flag = false;
     memset_num_threads = get_memset_num_threads(smp_cpus);
+    numpages_per_thread = g_new0(size_t, memset_num_threads);
     memset_thread = g_new0(MemsetThread, memset_num_threads);
-    numpages_per_thread = (numpages / memset_num_threads);
-    size_per_thread = (hpagesize * numpages_per_thread);
+    calc_page_per_thread(numpages, memset_num_threads, numpages_per_thread);
+
     for (i = 0; i < memset_num_threads; i++) {
         memset_thread[i].addr = addr;
-        memset_thread[i].numpages = (i == (memset_num_threads - 1)) ?
-                                    numpages : numpages_per_thread;
+        memset_thread[i].numpages = numpages_per_thread[i];
         memset_thread[i].hpagesize = hpagesize;
         qemu_thread_create(&memset_thread[i].pgthread, "touch_pages",
                            do_touch_pages, &memset_thread[i],
                            QEMU_THREAD_JOINABLE);
-        addr += size_per_thread;
-        numpages -= numpages_per_thread;
+        addr += numpages_per_thread[i] * hpagesize;
+        numpages -= numpages_per_thread[i];
     }
+    thread_create_flag = true;
+    qemu_cond_broadcast(&page_cond);
+
     for (i = 0; i < memset_num_threads; i++) {
         qemu_thread_join(&memset_thread[i].pgthread);
     }
-- 
1.8.3.1