sessionctx: fix tidb_gogc_tuner_threshold

Makefile

@@ -415,7 +415,7 @@ bazel_coverage_test: failpoint-enable bazel_ci_prepare
 
 bazel_build: bazel_ci_prepare
 	mkdir -p bin
-	bazel $(BAZEL_GLOBAL_CONFIG) build --remote_download_minimal $(BAZEL_CMD_CONFIG) \
+	bazel $(BAZEL_GLOBAL_CONFIG) build $(BAZEL_CMD_CONFIG) \
 		//... --//build:with_nogo_flag=true
 	bazel $(BAZEL_GLOBAL_CONFIG) build $(BAZEL_CMD_CONFIG) \
 		//cmd/importer:importer //tidb-server:tidb-server //tidb-server:tidb-server-check --//build:with_nogo_flag=true

sessionctx/variable/sysvar.go

@@ -748,12 +748,20 @@ var defaultSysVars = []*SysVar{
 			}
 			return strconv.FormatFloat(floatValue, 'f', -1, 64), nil
 		},
-		SetGlobal: func(_ context.Context, s *SessionVars, val string) error {
+		SetGlobal: func(_ context.Context, s *SessionVars, val string) (err error) {
 			factor := tidbOptFloat64(val, DefTiDBGOGCTunerThreshold)
 			GOGCTunerThreshold.Store(factor)
 			memTotal := memory.ServerMemoryLimit.Load()
-			threshold := float64(memTotal) * factor
-			gctuner.Tuning(uint64(threshold))
+			if memTotal == 0 {
+				memTotal, err = memory.MemTotal()
+				if err != nil {
+					return err
+				}
+			}
+			if factor > 0 {
+				threshold := float64(memTotal) * factor
+				gctuner.Tuning(uint64(threshold))
+			}
 			return nil
 		},
 	},