libct/intelrdt: skip reading /proc/cpuinfo

Reading /proc/cpuinfo is a surprisingly expensive operation. Since
kernel version 4.12 [1], opening /proc/cpuinfo on an x86 system can
block for around 20 milliseconds while the kernel samples the current
CPU frequency. There is a very recent patch [2] which gets rid of the
delay, but has yet to make it into the mainline kenel. Regardless,
kernels for which opening /proc/cpuinfo takes 20ms will continue to be
run in production for years to come. libcontainer only opens
/proc/cpuinfo to read the processor feature flags so all the delays to
get an accurate snapshot of the CPU frequency are just wasted time.

If we wanted to, we could interrogate the CPU features directly from
userspace using the `CPUID` instruction. However, Intel and AMD CPUs
have flags in different positions for their analogous sub-features and
there are CPU quirks [3] which would need to be accounted for. Some
Haswell server CPUs support RDT/CAT but are missing the `CPUID` flags
advertising their support; the kernel checks for support on that
processor family by probing the the hardware using privileged
RDMSR/WRMSR instructions [4]. This sort of probing could not be
implemented in userspace so it would not be possible to check for RDT
feature support in userspace without false negatives on some hardware
configurations.

It looks like libcontainer reads the CPU feature flags as a kind of
optimization so that it can skip checking whether the kernel supports an
RDT sub-feature if the hardware support is missing. As the kernel only
exposes subtrees in the `resctrl` filesystem for RDT sub-features with
hardware and kernel support, checking the CPU feature flags is redundant
from a correctness point of view. Remove the /proc/cpuinfo check as it
is an optimization which actually hurts performance.

[1]: https://unix.stackexchange.com/a/526679
[2]: https://lore.kernel.org/all/20220415161206.875029458@linutronix.de/
[3]: https://github.com/torvalds/linux/blob/7cf6a8a17f5b134b7e783c2d45c53298faef82a7/arch/x86/kernel/cpu/resctrl/core.c#L834-L851
[4]: https://github.com/torvalds/linux/blob/a6b450573b912316ad36262bfc70e7c3870c56d1/arch/x86/kernel/cpu/resctrl/core.c#L111-L153

Signed-off-by: Cory Snider <csnider@mirantis.com>
(cherry picked from commit 9f10748)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/intelrdt/intelrdt.go

@@ -1,7 +1,6 @@
 package intelrdt
 
 import (
-	"bufio"
 	"bytes"
 	"errors"
 	"fmt"
@@ -199,40 +198,28 @@ func featuresInit() {
 			return
 		}
 
-		// 2. Check if hardware and kernel support Intel RDT sub-features.
-		flagsSet, err := parseCpuInfoFile("/proc/cpuinfo")
-		if err != nil {
-			return
-		}
-
-		// 3. Double check if Intel RDT sub-features are available in
-		// "resource control" filesystem. Intel RDT sub-features can be
+		// 2. Check if Intel RDT sub-features are available in "resource
+		// control" filesystem. Intel RDT sub-features can be
 		// selectively disabled or enabled by kernel command line
 		// (e.g., rdt=!l3cat,mba) in 4.14 and newer kernel
-		if flagsSet.CAT {
-			if _, err := os.Stat(filepath.Join(root, "info", "L3")); err == nil {
-				catEnabled = true
-			}
+		if _, err := os.Stat(filepath.Join(root, "info", "L3")); err == nil {
+			catEnabled = true
 		}
-		if flagsSet.MBA {
-			if _, err := os.Stat(filepath.Join(root, "info", "MB")); err == nil {
-				mbaEnabled = true
-			}
+		if _, err := os.Stat(filepath.Join(root, "info", "MB")); err == nil {
+			mbaEnabled = true
 		}
-		if flagsSet.MBMTotal || flagsSet.MBMLocal || flagsSet.CMT {
-			if _, err := os.Stat(filepath.Join(root, "info", "L3_MON")); err != nil {
-				return
-			}
-			enabledMonFeatures, err = getMonFeatures(root)
-			if err != nil {
-				return
-			}
-			if enabledMonFeatures.mbmTotalBytes || enabledMonFeatures.mbmLocalBytes {
-				mbmEnabled = true
-			}
-			if enabledMonFeatures.llcOccupancy {
-				cmtEnabled = true
-			}
+		if _, err := os.Stat(filepath.Join(root, "info", "L3_MON")); err != nil {
+			return
+		}
+		enabledMonFeatures, err = getMonFeatures(root)
+		if err != nil {
+			return
+		}
+		if enabledMonFeatures.mbmTotalBytes || enabledMonFeatures.mbmLocalBytes {
+			mbmEnabled = true
+		}
+		if enabledMonFeatures.llcOccupancy {
+			cmtEnabled = true
 		}
 	})
 }
@@ -286,58 +273,6 @@ func Root() (string, error) {
 	return intelRdtRoot, intelRdtRootErr
 }
 
-type cpuInfoFlags struct {
-	CAT bool // Cache Allocation Technology
-	MBA bool // Memory Bandwidth Allocation
-
-	// Memory Bandwidth Monitoring related.
-	MBMTotal bool
-	MBMLocal bool
-
-	CMT bool // Cache Monitoring Technology
-}
-
-func parseCpuInfoFile(path string) (cpuInfoFlags, error) {
-	infoFlags := cpuInfoFlags{}
-
-	f, err := os.Open(path)
-	if err != nil {
-		return infoFlags, err
-	}
-	defer f.Close()
-
-	s := bufio.NewScanner(f)
-	for s.Scan() {
-		line := s.Text()
-
-		// Search "cat_l3" and "mba" flags in first "flags" line
-		if strings.HasPrefix(line, "flags") {
-			flags := strings.Split(line, " ")
-			// "cat_l3" flag for CAT and "mba" flag for MBA
-			for _, flag := range flags {
-				switch flag {
-				case "cat_l3":
-					infoFlags.CAT = true
-				case "mba":
-					infoFlags.MBA = true
-				case "cqm_mbm_total":
-					infoFlags.MBMTotal = true
-				case "cqm_mbm_local":
-					infoFlags.MBMLocal = true
-				case "cqm_occup_llc":
-					infoFlags.CMT = true
-				}
-			}
-			return infoFlags, nil
-		}
-	}
-	if err := s.Err(); err != nil {
-		return infoFlags, err
-	}
-
-	return infoFlags, nil
-}
-
 // Gets a single uint64 value from the specified file.
 func getIntelRdtParamUint(path, file string) (uint64, error) {
 	fileName := filepath.Join(path, file)