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Re: [PATCH v2] target/ppc/kvm: Cache timebase frequency
From: |
David Gibson |
Subject: |
Re: [PATCH v2] target/ppc/kvm: Cache timebase frequency |
Date: |
Mon, 22 Mar 2021 11:32:57 +1100 |
On Wed, Mar 17, 2021 at 06:57:07PM +0100, 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.
>
> Rename kvmppc_get_tbfreq() to kvmppc_get_tbfreq_procfs() and
> rename the 'retval' variable to make it clear it is used as
> fallback only. Come up with a new version of kvmppc_get_tbfreq()
> that calls kvmppc_get_tbfreq_procfs() only once and keep the
> value in a static.
>
> Zero is certainly not a valid value for the timebase frequency.
> Treat atoi() returning zero as another parsing error and return
> the fallback value instead. This allows kvmppc_get_tbfreq() to
> use zero as an indicator that kvmppc_get_tbfreq_procfs() hasn't
> been called yet.
>
> With this patch applied:
>
> 384 0.518382
>
> Signed-off-by: Greg Kurz <groug@kaod.org>
Applied to ppc-for-6.0, thanks.
> ---
> v2: - do the caching in a distinct function for clarity (Philippe)
> - rename 'retval' to 'tbfreq_fallback'
> - expand the changelog a bit
> ---
> target/ppc/kvm.c | 25 +++++++++++++++++++------
> 1 file changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 298c1f882c67..104a308abb57 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -1815,24 +1815,37 @@ 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;
> - uint32_t retval = NANOSECONDS_PER_SECOND;
> + uint32_t tbfreq_fallback = NANOSECONDS_PER_SECOND;
> + uint32_t tbfreq_procfs;
>
> if (read_cpuinfo("timebase", line, sizeof(line))) {
> - return retval;
> + return tbfreq_fallback;
> }
>
> ns = strchr(line, ':');
> if (!ns) {
> - return retval;
> + return tbfreq_fallback;
> }
>
> - ns++;
> + tbfreq_procfs = atoi(++ns);
> +
> + /* 0 is certainly not acceptable by the guest, return fallback value */
> + return tbfreq_procfs ? tbfreq_procfs : tbfreq_fallback;
> +}
> +
> +uint32_t kvmppc_get_tbfreq(void)
> +{
> + static uint32_t cached_tbfreq;
> +
> + if (!cached_tbfreq) {
> + cached_tbfreq = kvmppc_get_tbfreq_procfs();
> + }
>
> - return atoi(ns);
> + return cached_tbfreq;
> }
>
> bool kvmppc_get_host_serial(char **value)
>
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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