Re: [PATCH v1 2/4] KVM: stats: Update doc for histogram statistics

[David Matlack <dmatlack@google.com>]

On Thu, Jul 08, 2021 at 09:14:38PM +0000, David Matlack wrote:
> On Tue, Jul 06, 2021 at 06:03:48PM +0000, Jing Zhang wrote:
> > Add documentations for linear and logarithmic histogram statistics.
> > Add binary stats capability text which is missing during merge of
> > the binary stats patch.
> > 
> > Signed-off-by: Jing Zhang <jingzhangos@google.com>
> > ---
> >  Documentation/virt/kvm/api.rst | 36 +++++++++++++++++++++++++++++++---
> >  1 file changed, 33 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 3b6e3b1628b4..948d33c26704 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -5171,6 +5171,9 @@ by a string of size ``name_size``.
> >  	#define KVM_STATS_TYPE_CUMULATIVE	(0x0 << KVM_STATS_TYPE_SHIFT)
> >  	#define KVM_STATS_TYPE_INSTANT		(0x1 << KVM_STATS_TYPE_SHIFT)
> >  	#define KVM_STATS_TYPE_PEAK		(0x2 << KVM_STATS_TYPE_SHIFT)
> > +	#define KVM_STATS_TYPE_LINEAR_HIST	(0x3 << KVM_STATS_TYPE_SHIFT)
> > +	#define KVM_STATS_TYPE_LOG_HIST		(0x4 << KVM_STATS_TYPE_SHIFT)
> > +	#define KVM_STATS_TYPE_MAX		KVM_STATS_TYPE_LOG_HIST
> >  
> >  	#define KVM_STATS_UNIT_SHIFT		4
> >  	#define KVM_STATS_UNIT_MASK		(0xF << KVM_STATS_UNIT_SHIFT)
> > @@ -5178,11 +5181,13 @@ by a string of size ``name_size``.
> >  	#define KVM_STATS_UNIT_BYTES		(0x1 << KVM_STATS_UNIT_SHIFT)
> >  	#define KVM_STATS_UNIT_SECONDS		(0x2 << KVM_STATS_UNIT_SHIFT)
> >  	#define KVM_STATS_UNIT_CYCLES		(0x3 << KVM_STATS_UNIT_SHIFT)
> > +	#define KVM_STATS_UNIT_MAX		KVM_STATS_UNIT_CYCLES
> >  
> >  	#define KVM_STATS_BASE_SHIFT		8
> >  	#define KVM_STATS_BASE_MASK		(0xF << KVM_STATS_BASE_SHIFT)
> >  	#define KVM_STATS_BASE_POW10		(0x0 << KVM_STATS_BASE_SHIFT)
> >  	#define KVM_STATS_BASE_POW2		(0x1 << KVM_STATS_BASE_SHIFT)
> > +	#define KVM_STATS_BASE_MAX		KVM_STATS_BASE_POW2
> 
> Should these FOO_MAX additions go in a separate commit too?
> 
> >  
> >  	struct kvm_stats_desc {
> >  		__u32 flags;
> > @@ -5214,6 +5219,22 @@ Bits 0-3 of ``flags`` encode the type:
> >      represents a peak value for a measurement, for example the maximum number
> >      of items in a hash table bucket, the longest time waited and so on.
> >      The corresponding ``size`` field for this type is always 1.
> > +  * ``KVM_STATS_TYPE_LINEAR_HIST``
> > +    The statistics data is in the form of linear histogram. The number of
> > +    buckets is specified by the ``size`` field. The size of buckets is specified
> > +    by the ``hist_param`` field. The range of the Nth bucket (1 <= N < ``size``)
> 
> Using 1-based indexes is a little jarring but maybe that's just me :).
> 
> > +    is [``hist_param``*(N-1), ``hist_param``*N), while the range of the last
> > +    bucket is [``hist_param``*(``size``-1), +INF). (+INF means positive infinity
> > +    value.) The bucket value indicates how many times the statistics data is in
> > +    the bucket's range.
> > +  * ``KVM_STATS_TYPE_LOG_HIST``
> > +    The statistics data is in the form of logarithmic histogram. The number of
> > +    buckets is specified by the ``size`` field. The base of logarithm is
> > +    specified by the ``hist_param`` field. The range of the Nth bucket (1 < N <
> 
> Should 1 be 2 here? The first bucket uses a slightly differen formula as
> you mention in the next sentence.

Oops I see you used "<" instead of "<=" so what you have is correct. I
think reordering the sentences would still be helpfull though.

> 
> Actually it may be clearer if you re-ordered the sentences a bit:
> 
>   The range of the first bucket is [0, 1), while the range of the last
>   bucket is [pow(hist_param, size-1), +INF). Otherwise the Nth bucket
>   (1-indexed) covers [pow(hist_param, N-2), pow(histparam, N-1)).
> 
> > +    ``size``) is [pow(``hist_param``, N-2), pow(``hist_param``, N-1)). The range
> > +    of the first bucket is [0, 1), while the range of the last bucket is
> > +    [pow(``hist_param``, ``size``-2), +INF). The bucket value indicates how many
> > +    times the statistics data is in the bucket's range.
> >  
> >  Bits 4-7 of ``flags`` encode the unit:
> >    * ``KVM_STATS_UNIT_NONE``
> > @@ -5246,9 +5267,10 @@ unsigned 64bit data.
> >  The ``offset`` field is the offset from the start of Data Block to the start of
> >  the corresponding statistics data.
> >  
> > -The ``unused`` field is reserved for future support for other types of
> > -statistics data, like log/linear histogram. Its value is always 0 for the types
> > -defined above.
> > +The ``hist_param`` field is used as a parameter for histogram statistics data.
> > +For linear histogram statistics data, it indicates the size of a bucket. For
> > +logarithmic histogram statistics data, it indicates the base of the logarithm.
> > +Only base of 2 is supported fo logarithmic histogram.
> 
> s/fo/for/
> 
> >  
> >  The ``name`` field is the name string of the statistics data. The name string
> >  starts at the end of ``struct kvm_stats_desc``.  The maximum length including
> > @@ -7182,3 +7204,11 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
> >  of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
> >  the hypercalls whose corresponding bit is in the argument, and return
> >  ENOSYS for the others.
> > +
> > +8.35 KVM_CAP_STATS_BINARY_FD
> > +----------------------------
> > +
> > +:Architectures: all
> > +
> > +This capability indicates the feature that userspace can get a file descriptor
> > +for every VM and VCPU to read statistics data in binary format.
> > -- 
> > 2.32.0.93.g670b81a890-goog
> >