Income taxes - Components of provision (benefit) for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Current:
U.S. Federal	$ 1,366	$ 0	$ 0
U.S. State	1,132	177	62
Foreign	1,463	816	(3,791)
Total current	3,961	993	(3,729)
Deferred:
U.S. Federal	0	(168)	0
U.S. State	0	0	0
Foreign	(271)	231	1,438
Total deferred	(271)	63	1,438
Provision for (benefit from) income taxes	$ 3,690	$ 1,056	$ (2,291)