Avoid expensive log.Valuer evaluation for disallowed level

[damnever]

A lot of failed operations in memcach_client.SetAsync will consume a lot of CPU:

goos: darwin
goarch: arm64
pkg: github.com/cortexproject/cortex/pkg/util/log
                      │   old.txt    │               new.txt               │
                      │    sec/op    │   sec/op     vs base                │
DisallowedLogLevel-10   1594.5n ± 6%   418.2n ± 1%  -73.77% (p=0.000 n=20)

                      │   old.txt   │              new.txt               │
                      │    B/op     │    B/op     vs base                │
DisallowedLogLevel-10   1567.0 ± 0%   623.0 ± 0%  -60.24% (p=0.000 n=20)

                      │  old.txt   │              new.txt               │
                      │ allocs/op  │ allocs/op   vs base                │
DisallowedLogLevel-10   23.00 ± 0%   13.00 ± 0%  -43.48% (p=0.000 n=20)

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[alanprot]

This is not being used anymore?

[damnever]

This is used by other packages.

[alanprot]

Can you explain what is the functional change here? are we missing/changing something?

[damnever]

@alanprot I agree, the change is simple but not easy to understand, the old code is like this:

logger0 := NewXXXLogger(...)
logger1 := level.NewFilter(logger0, lvl)
logger2 := log.With(logger1, "ts", log.DefaultTimestampUTC, "caller", log.Caller(3))

the new code is like this:

logger0 := NewXXXLogger(...)
logger1 := log.With(logger0, "ts", log.DefaultTimestampUTC, "caller", log.Caller(5))
logger2 := level.NewFilter(logger1, lvl)

the order matters since the logger2.Log does level filtering first then call the logger1.Log on demand, the logger1.Log evaluates the log.Valuer such as log.Caller..

I have add the reference link in the comments: go-kit/log#14 (comment)

[alanprot]

NIce! Ok..

[alvinlin123]

This is an awesome enhancement, thank you so much!