LDD3 notes: Passage of time

My notes while reading Linux Device Drivers 3rd edition


.Jiffies

  • get_jiffies_64()
  • comparison macros: time_after/before
  • jiffies <-> timeval / timespec
  • get_cycles()

short busy waits

  • n/u/mdelay()

1ms/1s resolution sleeps

  • msleep(), ssleep()

Working with 1/HZ resolution sleeps

  • wait_event_timeout() - returns timeout left, never negative
  • set_current_state(interruptible), schedule_timeout()
  • in_atomic(), in_interrupt()


Sleeping with waitqueues

  • macro: one init_waitqueue_head, multiple wait_event(_interruptible), one wake_up to wake up them all.
  • manual wait_event alternative: prepare_to_wait(), schedule(), finish_wait(), signal_pending()
  • 'exclusive' sleepers are woken up as specified batches or individually, they won't behave like a herd
  • going to schedule soon, use wake_up_onterruptible_sync, it won't reschedule right away
  • never use sleep_on, it's broken w/ race condition

Tasklets

  • atomic context in timer interrupt handler or softirq
  • multiple schedules result in single execution
  • cannot assume process context or access user space memory

Workqueue

  • queue has a kernel process, can sleep, but cannot access user space
  • each work_struct in queue only once at a time
  • can be delayed for jiffies


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