RCU

Reading a few LWN articles on RCUs (1 2 3) really shed some light on their properties and use.

On read-intensive scenarios, it's more efficient to replace read/write locking with an RCU. This is possible because all Linux platforms have atomic pointer read/write operations.

Read critical section
  • dereferences pointers through a mechanism with platform specific memory ordering guarantees: rcu_dereference()
  • may not sleep
  • may not keep or pass dereferenced pointers outside the critical section
Write critical section
  • Protects agains concurrent writes using regular spinlock mutex
  • Makes a copy of the original structure, making updates in the copy
  • Swaps in the new version, while still keeping the old version (atomic)
  • Invokes synchronize_rcu() to wait for all readers to exit their current rcu read critical sections.
  • ( A non-pre-emptible kernel can simpy wait for all CPUs to switch contexts. )
  • Free the old version
Properly used RCUs are immune to deadlocks, but synchronous writes may be very slow due to need to wait for the context switches.

Comments

Post a Comment

Popular posts from this blog

iMovie event library on a network drive, NAS

I was shocked to find that iMovie '09 would not allow storing event libraries on networked disks. The AFP volume would show up, but remain non-functional with a yellow exclamation sign. There are plenty of instructions for hacking around the limitation with clumsy symbolic links. I discovered a much simpler approach, using an undocumented (?) setting built right into iMovie:"Allow Network Volumes". Use at your own risk, try the following command in Terminal defaults write -app iMovie allowNV -bool true
Read more

Proxmox PCIe passthrough on HP gen8 - failed to set iommu for container

Problem Setting up PCIe passthrough from host to a VM was supposed to be easy. However, being an HP server, there was a bit more to it than usual. The VM simply refused to start when configured use Nvidia GPU from the host: vfio error: 0000:04:00.0: failed to setup container for group 21: failed to set iommu for container: Operation not permitted In dmesg there was a bit more background on what was wrong: fio-pci 0000:04:00.1: Device is ineligible for IOMMU domain attach due to platform RMRR requirement.  Contact your platform vendor. Luckily, HP had issued a customer advisory on this. It describes a convoluted method to disable this RMRR per slot basis. It seems to work for me, so I thought I'd write down some notes if I ever run into this again. Basic setup Proxmox has decent instructions for preparing the host for passthrough setup in general, in summary: -  add  intel_iommu=on to   GRUB_CMDLINE_LINUX_DEFAULT  in the file  /etc/default/grub - add vfio modules to  /etc/modules
Read more

Backup and restore observium

Note to self, how to backup and restore an Observium installation such as Turn Key Linux . backup mysqldump --all--databases >sql.txt tar zcvf rrd.tgz /opt/observium/rrd restore log in at https://observium:12322/ as adminer import the sql.txt tar zxvf  rrd.tgz -C /
Read more