Re: [PATCH] target/ppc/kvm: Cache timebase frequency

[Philippe Mathieu-Daudé]

On 3/17/21 4:24 PM, Greg Kurz wrote:
> Each vCPU core exposes its timebase frequency in the DT. When running
> under KVM, this means parsing /proc/cpuinfo in order to get the timebase
> frequency of the host CPU.
> 
> The parsing appears to slow down the boot quite a bit with higher number
> of cores:
> 
> # of cores     seconds spent in spapr_dt_cpus()
>       8                  0.550122
>      16                  1.342375
>      32                  2.850316
>      64                  5.922505
>      96                  9.109224
>     128                 12.245504
>     256                 24.957236
>     384                 37.389113
> 
> The timebase frequency of the host CPU is identical for all
> cores and it is an invariant for the VM lifetime. Cache it
> instead of doing the same expensive parsing again and again.
> 
> With this patch applied:
> 
>     384                 0.518382
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  target/ppc/kvm.c |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 298c1f882c67..9ad3dae29132 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -1819,7 +1819,13 @@ uint32_t kvmppc_get_tbfreq(void)
>  {
>      char line[512];
>      char *ns;
> -    uint32_t retval = NANOSECONDS_PER_SECOND;
> +    static uint32_t retval = -1;

Please document why in the code ...

> +
> +    if (retval != -1) {
> +        return retval;
> +    }
> +
> +    retval = NANOSECONDS_PER_SECOND;
>  
>      if (read_cpuinfo("timebase", line, sizeof(line))) {
>          return retval;
> @@ -1832,7 +1838,8 @@ uint32_t kvmppc_get_tbfreq(void)
>  
>      ns++;
>  
> -    return atoi(ns);
> +    retval = atoi(ns);
> +    return retval;
>  }

... or alternatively use self-documented code:

-- >8 --
diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 298c1f882c6..2b2fe5d8148 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -1815,7 +1815,7 @@ static int read_cpuinfo(const char *field, char
*value, int len)
     return ret;
 }

-uint32_t kvmppc_get_tbfreq(void)
+static uint32_t kvmppc_get_tbfreq_procfs(void)
 {
     char line[512];
     char *ns;
@@ -1835,6 +1835,17 @@ uint32_t kvmppc_get_tbfreq(void)
     return atoi(ns);
 }

+uint32_t kvmppc_get_tbfreq(void)
+{
+    static uint32_t cached_tbfreq;
+
+    if (!cached_tbfreq) {
+        cached_tbfreq = kvmppc_get_tbfreq_procfs();
+    }
+
+    return cached_tbfreq;
+}
+
 bool kvmppc_get_host_serial(char **value)
 {
     return g_file_get_contents("/proc/device-tree/system-id", value, NULL,
---