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 <gr...@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,
---
