linux-kernel - NFS dentry caching regression? was Re: [GIT PULL] Please pull NFS client updates for Linux 6.19

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Message-ID: <aUJ4rjyAOW3EWC-k@infradead.org>
Date: Wed, 17 Dec 2025 01:32:30 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Trond Myklebust <trondmy@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	linux-fsdevel@...r.kernel.org
Subject: NFS dentry caching regression? was Re: [GIT PULL] Please pull NFS
 client updates for Linux 6.19

Hi all,

the merge of this branch causes NFS lookup operation to shoot up a lot
for me.  And with merge I mean merge - both parent of the merge on their
own are fine.

With the script below that simulates running python scripts with lots
of imports that was created to benchmark delegation performance, the
number of lookups in the measurement period shoots up from 4 to about
410000, which is a bit suboptimal.  I have no idea how this could
happen, but it must be related to some sort of pathname lookup changes
I guess.  Other operations looks roughly the same.

---
#!/usr/bin/env bash
set -euo pipefail

if [ $# -ne 1 ]; then
    echo "Usage: $0 <NFS_MOUNT_PATH>"
    exit 1
fi

NFS_MOUNT="$1"
WARMUP_FILE_COUNT="8000"
RUNS=200
MODULE_COUNT=200
SAVEFILE="/tmp/nfsstat.bak"

echo "=== NFS delegation benchmark ==="
echo "NFS mount:          $NFS_MOUNT"
echo "Warmup file count:  $WARMUP_FILE_COUNT"
echo "Number of runs:     $RUNS"
echo "Module count:       $MODULE_COUNT"
echo


################################################################################
# Step 1: Create temporary directory on NFS
################################################################################
TEST_DIR=$(mktemp -d "$NFS_MOUNT/test_deleg_bench.XXXXXX")
MODULE_DIR="$TEST_DIR/pymods"
mkdir -p "$MODULE_DIR/delegtest"
MODDIR_INIT="$MODULE_DIR/delegtest/__init__.py"

cat > "$MODDIR_INIT" <<EOF
import os
import glob

file_paths = glob.glob(os.path.join(os.path.dirname(__file__), "*.py"))

__all__ = [
    os.path.basename(f)[:-3] 
    for f in file_paths 
    if os.path.isfile(f) and not f.endswith("__init__.py")
]
EOF

echo "[1] Creating $WARMUP_FILE_COUNT tiny files to accumulate delegations..."
mkdir -p "$TEST_DIR/fill"
for i in $(seq 1 "$WARMUP_FILE_COUNT"); do
    echo "f$i" > "$TEST_DIR/fill/file_$i"
done

echo "[1] Warmup delegation files created."

################################################################################
# Step 2: Create many tiny Python modules to exercise import workload
################################################################################
echo "[2] Creating $MODULE_COUNT dummy python modules..."
for i in $(seq 1 "$MODULE_COUNT"); do
    echo "x = $i" > "$MODULE_DIR/delegtest/mod$i.py"
done
#umount -o remount $NFS_MOUNT

# Python snippet:
# repeatedly import all modN modules; measure iterations completed
BENCH_SCRIPT="$TEST_DIR/bench.py"

cat > "$BENCH_SCRIPT" <<EOF
import sys

sys.path.insert(0, "$MODULE_DIR")

from delegtest import *
EOF

################################################################################
# Step 3: Pre-benchmark NFS client counters
################################################################################
echo "[3] Capturing baseline NFS client stats..."
sync $NFS_MOUNT
#mount -o remount $NFS_MOUNT
cp /proc/net/rpc/nfs $SAVEFILE

################################################################################
# Step 4: Run Python benchmark
################################################################################
echo "[4] Running Python import benchmark..."

time {
for i in $(seq 1 "$RUNS"); do
    python3 "$BENCH_SCRIPT"
done
}

################################################################################
# Step 5: Produce NFS client delta report
################################################################################
echo
echo "=== NFS client DELTA REPORT ==="
echo

nfsstat -c -S $SAVEFILE  -l
rm -f $SAVEFILE

echo
echo "Test directory:       $TEST_DIR"
echo
echo "=== Done ==="